Shut down cleanly when phone is shutting down or memory is low

Closes #1190

See merge request akwizgran/briar!742

Looks like I added the last commit after the MR was merged.

Shorten and clean up various strings, remove empty forum warning bubble

See merge request akwizgran/briar!741

Hide UI during shutdown

See merge request akwizgran/briar!737

Add denormalised columns to messageDependencies table

See merge request akwizgran/briar!733

Add fullscreen button to QR code view

Closes #346

See merge request akwizgran/briar!734

Show different notification settings for Android O

Closes #1159

See merge request akwizgran/briar!727

Use WifiManager to get wifi network information

Closes #845

See merge request akwizgran/briar!743

Discard tasks submitted to ScheduledExecutorService during shutdown

Closes #1184

See merge request akwizgran/briar!739

This ensures we bind to the wifi interface even if it doesn't have internet access and there's another interface with internet access (e.g. mobile data).

Remove mention of pen icon from forum empty state message

Closes #965

See merge request akwizgran/briar!740

Show viewfinder again after connection fails

Closes #1177

See merge request akwizgran/briar!735

This also makes the defaults consistent with Android versions below O.

Remove client ID from validator's DB methods

See merge request akwizgran/briar!732

This causes the validator to treat the dependent message in the same way regardless of whether there is a subscription to the dependency's group.

Disable settings until they have been loaded

Closes #1169

See merge request akwizgran/briar!726

In practise, this is not noticeable in the UI.
Only when the database is congested, it should become visible and
prevent a crash when the sound setting is clicked.