Add support for 64-bit Tor binaries

Closes #1506

See merge request briar/briar!1161

[android] Raise minimum API level to 16

Closes #1633

See merge request briar/briar!1171

Fix group sharing message deletion test

See merge request briar/briar!1168

Remove old migration code for Tor settings

See merge request briar/briar!1172

This shouldn't be merged before raising the minimum
API version to 16.

Allow sharer to re-share a shareable again after leaving

Closes #1632

See merge request briar/briar!1169

Restore custom layout behaviour for handling snackbar

Closes #1582

See merge request briar/briar!1170

Delete conversation messages belonging to completed private group sessions

Closes #1627

See merge request briar/briar!1167

Delete conversation messages belonging to completed sharing sessions

See merge request briar/briar!1164

Fix MessageTracker group counts after deleting messages

See merge request briar/briar!1166

Add feature flag for private message deletion

See merge request briar/briar!1165

Delete conversation messages belonging to completed introduction sessions

See merge request briar/briar!1163

Also remove stale comment

A session is completed if it returned to the START state
and if all sent messages have been ACKed by the receiver.

The session's metadata is kept in case the user restarts the session
by doing another introduction.

Add conversation menu action to delete all messages

Closes #1626

See merge request briar/briar!1159