Add setting for opting in or out of Tor bridges

It would make sense to have a way of configuring whether tor use bridges (and all the machinery that goes into deciding the default setting for a given country)

added Mailbox Weight: 10 labels

added Mailbox app label

assigned to @grote

!103 (merged) adds automatic bridge selection

mentioned in issue #119

added Sponsor 6 label

UI Plan:

Tor circumvention settings

Automatic bridge selection [ ]

Bridges get chosen based on your location

Use Bridges [ ]

Bridge types (disabled/hidden when automatic is checked)

Vanilla [ ]

Obfs4 [ ]

Snowflake [ ]

Meek [ ]

mentioned in merge request !169 (merged)

added S: in code review label

mentioned in commit a3ef16e6

closed with merge request !169 (merged)