Update Mastodon to Rails 6.1

[ClearlyClaire]

PRs to merge first:

• Prepare Mastodon for Rails 6 (Prepare Mastodon for Rails 6 #15911)
• Improve account counters handling (Improve account counters handling #15913)
• Further preparation for Rails 6 (Further preparation for Rails 6 #15916)
• Prepare Mastodon for zeitwerk autoloader (Prepare Mastodon for zeitwerk autoloader #15917)

PRs to consider first:

• Use more robust hook for loading timestamp_id function into database (Use more robust hook for loading timestamp_id function into database #15919)

This is work in progress, some tests are still failing:

• the code in increment_count! to handle race conditions isn't working correctly. Addressed by Improve account counters handling #15913
• the tests about the force_ssl config do not work anymore, i am not sure how to test those
• migrations still fail (partly because of ActiveRecord migrations with polymorphic references create indexes with different name rails/rails#41693, worked around)

Other broken things to look fix:

• the hooks surrounding database setup seem to have changed somehow. Addressed by Use more robust hook for loading timestamp_id function into database #15919

Nice to have:

• enable zeitwerk (new autoloading code, more deterministic and supposedly faster): see Prepare Mastodon for zeitwerk autoloader #15917

This PR also does not switch to the new framework defaults yet, this means, among other things, that zeitwerk isn't enabled yet, and will require more work.

[ClearlyClaire]

To elaborate on the “hooks surrounding database setup seem to have changed somehow”, when fiddling with my test setup, I often get the following:

PG::UndefinedFunction: ERROR: function timestamp_id(text) does not exist
LINE 1: ...E TABLE "encrypted_messages" ("id" bigint DEFAULT timestamp_...

This doesn't seem to occur in the CI, but still, something is off and I need to find what before this PR is ready.

[ClearlyClaire]

Marked as ready for review, as I'm fairly happy with how it looks now, with the following caveats:

• it hasn't been tested in a production environment yet
• I'm not super happy with having to switch with unreleased forks for a few gems, but they are currently unmaintained

[ClearlyClaire]

I have found no regression so far, but clearing the Rails cache is required as some of the stored objects are Rails models that don't load quite the same on Rails 6.1.

[zunda]

As ClearlyClaire wrote, this change seems to make the data in Rails' cache inconsistent with the code resulting with errors like below. Clearing the cache with

tootctl cache clear

seems to have resolved the inconsistency.

method=GET path=/users/zundan/statuses/105946269594904129 format=json controller=StatusesController action=show status=500 error='ActionController::UrlGenerationError: No route matches {:action=>"show", :controller=>"emojis", :id=>#<CustomEmoji id: 183, shortcode: "saba", domain: nil, image_file_name: "food_kandume_saba.png", image_content_type: "image/png", image_file_size: 49717, image_updated_at: "2017-10-07 00:27:30.774446000 +0000", created_at: "2017-09-29 01:28:38.632698000 +0000", updated_at: "2017-10-07 00:27:31.124663000 +0000", disabled: false, uri: nil, image_remote_url: nil, visible_in_picker: true, category_id: nil, image_storage_schema_version: nil>}, possible unmatched constraints: [:id]
Did you mean?  emoji_path
new_admin_custom_emoji_url
new_admin_custom_emoji_path
batch_admin_custom_emojis_url' duration=519.68 view=0.00 db=224.12 key=acct:ClworldF@md.ggtea.org

ActionController::UrlGenerationError (No route matches {:action=>"show", :controller=>"emojis", :id=>#<CustomEmoji id: 183, shortcode: "saba", domain: nil, image_file_name: "food_kandume_saba.png", image_content_type: "image/png", image_file_size: 49717, image_updated_at: "2017-10-07 00:27:30.774446000 +0000", created_at: "2017-09-29 01:28:38.632698000 +0000", updated_at: "2017-10-07 00:27:31.124663000 +0000", disabled: false, uri: nil, image_remote_url: nil, visible_in_picker: true, category_id: nil, image_storage_schema_version: nil>}, possible unmatched constraints: [:id]
Did you mean?  emoji_path
new_admin_custom_emoji_url
new_admin_custom_emoji_path
batch_admin_custom_emojis_url):

app/lib/activitypub/tag_manager.rb:43:in `uri_for'
app/serializers/activitypub/emoji_serializer.rb:13:in `id'
app/lib/activitypub/serializer.rb:36:in `serializable_hash'
app/lib/activitypub/serializer.rb:36:in `serializable_hash'
app/lib/activitypub/adapter.rb:42:in `serializable_hash'
app/controllers/concerns/cache_concern.rb:22:in `render_with_cache'
app/controllers/statuses_controller.rb:38:in `block (2 levels) in show'
app/controllers/statuses_controller.rb:29:in `show'