[feature] Allow users to export data via the settings panel

[tsmethurst]

Description

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This pull request implements exporting of user data as CSV files (with the same formatting used by Mastodon - I checked) via the settings panel. To support this, a new client API section "exports" has been added, as well as a few new database functions.

Draft while I add more tests.

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.