Support loading sites via yaml, make defined enrollment top level

[nbrownus]

This changes the + button on the top left. Tapping it will now pop a modal bottom sheet with a few options for creating a site.

From scratch - this is the old style type it all in and save
From file - this opens a file picker and allows you to select a nebula config.yaml file. Any applicable configuration will be imported into a new site.
Enroll with defined.net - this is the old enrollment screen that you used to get to from the settings screen.

Things to test

iOS:

• Saving a site from scratch still works, no funky stuff when toggling the site or refreshing site details.
• Saving a site from file, same story regarding asserting no funky stuff.
• Enroll with defined.net code still works.
• Deep linking enrollment with defined still works.

Android:

• Saving a site from scratch still works, no funky stuff when toggling the site or refreshing site details.
• Saving a site from file, same story regarding asserting no funky stuff.
• Enroll with defined.net code still works.
• Deep linking enrollment with defined still works.

Closes #135
Possible closes #152

[JackDoan]

I know we don't currently do firewall really on mobile, do you wanna explicitly reject it here?

[nbrownus]

I have a slight desire towards leaving it at ignored. At best it would be a type of warning saying "we ignored a thing you can't set by hand anyway" which doesn't seem super helpful and also open up questions around announcing ignoring other things we don't care about.

[JackDoan]

some notes, overall lgtm