Add additional modes to configure what's allowed during login / registration

[jryans]

We currently have a config option disable_custom_urls which means no custom HS for both login and registration.

In some environments, we'd like to disable custom HS for registration (since it simplifies account creation), but leave login open to any HS (so that more advanced users can still use any Riot with their existing account). Let's add a mode that supports this use case.

[jryans]

After discussing more with @lampholder, we'd like to offer a bit more flexibility, so that Riot for different use cases.

In particular, we'll add 3 independent settings:

• Hide the 3 column server type selector
• Disable custom HS for registration
• Disable custom HS for login