Draft: Add sign up system directly in the site

Ategonrequested to merge
feat/signups into main

Closes #244

Adds a new sign up modal that allows you to enter fields for signing up for a lemmy account and then it will automatically send a post request to an instance to make the account. Then it will send it to fediauth to authenticate instead of the user needing to do the manual code fetching from dms.

Useful to help reduce frontloaded information so that people participate rather than give up due to weird authentication at the start.

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