Fix and refactor rewritten IS url feature. Add sample config docs

[anoadragon453]

This PR:

• Changes dinsic config option rewrite_identity_server_urls to include protocol schemes (ex. https://)
  • @michaelkaye seems ok with this
  • tests concerning this are updated
• Renames every instance of a variable containing an identity server without a protocol scheme to id_server, and likewise with a protocol scheme to url
• Adds documentation about rewrite_identity_server_urls to the sample config file
• Consolidates all rewriting and "https://"-prepending into a single helper function

URLs/domain names of identity servers can either come from:

• Clients sending id_server params (no protocol scheme)
• account_threepid_delegates (includes scheme)

For the former, we use the add_https parameter to turn them into URLs.

I'm merging this to the release branch instead of dinsic as it concerns account_threepid_delegates stuff which is coming from mainline.

CI is failing as the various linting tools have not been run on this branch yet. Unfortunately we can't get to the rest of the tests without that though.

Not sure about adding a newspaper file...

[michaelkaye]

Does this mean rewrite_identity_server_urls should be a map of url prefix -> url prefix (no trailing /, i think) rather than host->host as it is currently? This is fine; just need to make sure we update the configuration appropriately.

[anoadragon453]

Does this mean rewrite_identity_server_urls should be a map of url prefix -> url prefix (no trailing /, i think) rather than host->host as it is currently? This is fine; just need to make sure we update the configuration appropriately.

Yes, exactly. No trailing slash.

[babolivier]

lgtm once these points are addressed. You also probably want to regenerate the sample config for consistency.