Respect registration config on OAuth signup flow

[thebaer]

This alters the current OAuth flow to respect an instance's user registration config. It includes:

• Closed registration. Now, if the OAuth flow would've normally led to the creation of a new user on a closed-registration instance, it is blocked instead and an error is returned to the user.
• Invite-based registration. Now, on an invite link, a user has the option of signing up via any configured OAuth provider.

This PR includes style and database changes. Run the following commands to update your development environment:

make ui
writefreely db migrate

[thebaer]

Added a few light improvements to the OAuth signup flow here, too:

• Skip random username generation if unnecessary
• Use "sign up" language on OAuth signup page, instead of "log in" language
• Inform user when they try to login to an OAuth-based account with a password but don't have one set