[feature] Implement following hashtags

[VyrCossont]

Description

• Adds API methods for following, unfollowing, and getting tags, and listing followed tags
• Sends new or updated public statuses with tags to the home timelines of local accounts following those tags

Closes #2326

Comments

Followed tags have no state of their own, so we only store the association between a tag ID and an account ID. Caches mappings from a tag to accounts that follow it, and from an account to all the tags that it follows.

Checklist

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have not leveraged AI to create the proposed changes.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[VyrCossont]

Hashtags I thought I'd followed on my test instance aren't showing up in my home timeline when they come in on remote statuses. Switching this to draft while I investigate.

[VyrCossont]

It was boost-related. Here's the situation:

• I follow someone but I hide their boosts because they're too noisy
• I still want to see any post that hits my instance with, say, #retrocomputing
• they boost a post from an account that I don't follow, original post has #retrocomputing
• the status actually being timelined in timelineAndNotifyStatus is the boost, not the original
• I don't see boosts from the account that I follow, so I don't see the boost
• the boost itself doesn't have any tags, so I don't see it due to following the hashtag

Fix: unwrap boosts when timelining based on post content.

[VyrCossont]

Apparently I need to explicitly check user-user blocks; the normal followers path checks them implicitly, because there can't be a follow relationship where a block exists. Admin-level domain blocks for remote statuses should be covered by enrichStatus.

[VyrCossont]

Fix is working. Hopefully that was the last missing bit.

[NyaaaWhatsUpDoc]

looks good to me! great work again 😎