[feature] Hashtag federation (in/out), hashtag client API endpoints

[tsmethurst]

Description

If this is a code change, please include a summary of what you've coded, and link to the issue(s) it closes/implements.

If this is a documentation change, please briefly describe what you've changed and why.

This pull request implements the following:

1. Federate hashtags in and and out of GoToSocial correctly.
2. Client API to view hashtag timeline.
3. Client API to search for hashtags.
4. Serve stub page at web timeline endpoint (pending discussion about what to actually show at these endpoints).
5. Document hashtag usage in the user guide, and add federation docs for hashtags.

closes #2019
closes #1822
closes #967

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• 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 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.

[NyaaaWhatsUpDoc]

finished reviewing this now. looks really great!! once the comments have been addressed, jokey ones aside :p, this should be ready to go!