[bugfix/frontend] Add nosubmit option to form fields; use it when instance custom CSS disabled

[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.

When custom CSS is not enabled on an instance, submitting any custom CSS update (even if it's just an empty string) returns a 400 error since custom CSS is not allowed.

Previously, we were getting around this by just not generating a form field at all for custom CSS when it was disabled on the instance. When I refactored some of the frontend stuff, I made it so the form field was shown, but was not enabled and couldn't be updated by users. This causes empty custom_css to be sent along to the instance when doing profile updates, meaning the update always fails.

To get around this, this PR adds a new nosubmit option to form hooks. If set to true, then the hook fields become 'internal' and won't be submitted as part of mutation data. This means they can still be shown, but become essentially "read only".

With this change, it's possible to update profiles on instances that don't have custom CSS enabled, again.

Change also fixes a couple very small bugs that cropped up while I was looking for the issue.

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.

[tsmethurst]

This little bugger was causing form update state to not get back to the button that submitted it properly, meaning errors were being swallowed.