[feature] Admin accounts endpoints; approve/reject sign-ups

[tsmethurst]

Description

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This pull request implements the rest of #703 by implementing functionality to allow admins to view pending account sign-ups, and approve or reject them.

• implements /api/v1/admin/accounts and /api/v2/admin/accounts endpoints, to allow admins to page through lists of accounts with various parameters (follows Mastodon API).
• implements /api/v1/admin/accounts/{id} endpoint, to allow admins to get the admin view of a specific account
• implements new settings panel sections for browsing through accounts, and for showing a list of pending accounts
• updates admin panel detailed account view with more stats about the account being viewed, and adds options to approve / reject sign-ups there
• documents sign-up flow for admins
• update various bits and bobs of the settings panel stuff, to use typescript a bit more

Closes #703

Checklist

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• 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.

[NyaaaWhatsUpDoc]

looking gooooooooood 🚀 👀