Show warning on status screen when device is offline

Closes #117

See merge request !94

Use app:srcCompat for inflating vector drawables

Closes #112 and #109

See merge request !93

Delete stale files periodically

Closes #102

See merge request !90

Document the order in which files are listed

See merge request !95

and prevent crashes

For now we delete stale files (older than 30 days) once a day.

Improve stopping and wiping behavior

Closes #60

See merge request !89

* Progress indicator for stopping
* Progress indicator for wiping
* More logging during startup and stopping for debugging
* Put post wiping fragment into separate process
* Improve exiting behavior on stop/wipe
* Make wiping fragment display transition to stopping once wiping is
  done (by changing the status text view)
* Make stopping and wiping take at least 1 second so that the user can
  perceive what's going on if these finish very quickly on fast devices
* When detecting on startup, that the app is already stopping or wiping,
  display the stopping/wiping fragment instead of failing silently
* Exit application when reaching STOPPED state, independent of lifecycle

Make status endpoint available to contacts

Closes #125

See merge request !91

Don't start on boot when app has been manually stopped before

Closes #87

See merge request !83

Flesh out stub of status screen

Closes #118, #92, and #41

See merge request !71

Also:
* Add fragment after wiping with further instructions
* Don't allow multiple MainActivity instances
* Finish activity when stopping mailbox

Limit the size of uploaded files

Closes #105

See merge request !86

Display different notification texts depending on status

Closes #86 and #79

See merge request !80

Include list of supported versions in setup response and add versions endpoint

Closes #103 and #104

See merge request !77

and rethrow exception

The mailbox won't allow clients to upload files larger than 1 MiB.

Replace last hard-coded string

Closes #96

See merge request !79

Also, add a few words to the dictionary and remove an unused string

Show more progress information during startup

Closes #110

See merge request !85

in preparation to add extra data to it

Port Briar's Base32 implementation for Java 6 compatibility

Closes #108

See merge request !82

Add CLI options for info/debug/trace logging

Closes #88

See merge request !78

Replace Material3 circular progress indicator which doesn't work reliably

Closes #111

See merge request !84

There are issues with the Material3 indicator on API levels 22 and 23.

Downgrade H2 to 1.4.192 for Java 6 compatibility

Closes #107

See merge request !81