Handle unchecking all bridges

!169 (comment 76780)

added Sponsor 6 label

added Weight: 3 label

added S: needs diagnosis label

added Mailbox Mailbox app labels

Current implementation plan: not allow unchecking the last bridge type and show a toast or snackbar that the user must at least have one bridge type enabled or uncheck use bridges.

assigned to @grote

removed S: needs diagnosis label

mentioned in merge request !174 (merged)

mentioned in commit a6d572c6

closed with merge request !174 (merged)