[feature] Account alias / move API + db models

[tsmethurst]

Description

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

This pull request adds client API support and documentation for setting account aliases ("alsoKnownAs") and triggering the account Move activity.

• add POST to /api/v1/account/alias to set/unset aliases.
• add POST to /api/v1/account/move to trigger a move to target account.
• update database models and support functions to set/unset/retrieve aka and move URIs properly
• update internal-to-frontend conversion logic to include moved and aka URIs information

Currently while it is possible to send a Move activity to the clientAPI processor via the newly-added endpoints, assuming all preconditions are met, nothing is yet processed. This will come in a subsequent PR.

relates to #130

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]

have left a few comments here and there but otherwise looks good to me! :)