[feature] Client API endpoints + v. basic web view for pinned posts

[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 client API endpoints to allow users to pin + unpin toots (maximum 10, currently), and to allow API callers to view pinned toots when getting an account's statuses.

It also makes some minor changes to our html templates and css to show a basic view of pinned toots at the top of a profile in the web view.

This PR does not yet include logic for federating pinned posts in and out of GoToSocial, that will be in a separate PR.

closes #947

Checklist

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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]

Some questions about implementation details so far, but otherwise looking generally good. Once we've finished talking through these I'll finish reviewing :)

[NyaaaWhatsUpDoc]

Surely this will panic if !paging, i.e. pinnedResp == nil. Would it be better to instead do var pinnedItems []any and set these within the !paging clause i.e. pinnedItem = pinnedResp.

[NyaaaWhatsUpDoc]

Wait just realized that item is always allocated. But this way might be a lil' neater anywho 😇

[NyaaaWhatsUpDoc]

2 final questions regarding expected behaviour just to double check this is expected behaviour that will work with existing clients. And one tiny nitpick.

But otherwise the code looks really good and once you have addressed those feel free to merge this :)