[feature] Parse instance descriptors as markdown, show T&C on /about

[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 updates our settings panel to allow parsing instance descriptor fields as markdown (short description, description, terms and conditions), presenting the original "unparsed" version of the field to the user for editing, similar to what we do for status text and account note/bio.

Also updates the /about page to add a table of contents (since it's getting long), and expose terms and conditions.

Few little other nice changes:

• Allow code formatting + syntax highlighting outside of just statuses.
• Allow contact account to be unset (closes [feature] Allow unsetting contact account #1311)
• Show character limits + do validation for descriptor fields.

Closes #143
Closes #707

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]

only a single comment, otherwise looks good :)