use viewmodel+livedata for loading stuff

mailbox-android/build.gradle

.../org/briarproject/mailbox/activity/ActivityComponent.java

...org/briarproject/mailbox/navdrawer/NavDrawerActivity.java

...a/org/briarproject/mailbox/overview/OverviewFragment.java

...a/org/briarproject/mailbox/settings/SettingsFragment.java

...ava/org/briarproject/mailbox/viewmodel/BaseViewModel.java

...roject/mailbox/viewmodel/MailboxOwnerStatusViewModel.java

...rg/briarproject/mailbox/viewmodel/ViewModelComponent.java

...a/org/briarproject/mailbox/viewmodel/ViewModelModule.java

...va/org/briarproject/mailbox/viewmodel/ViewModelScope.java