bridge settings

follow-up of !70 (merged)

closes #30 (closed) (well, the UI part, depending on tpo/core/onionmasq#54 (closed) for the underlying functionality)

closes #32 (closed)

Known bug in onionmasq: leads to a crash if you try to run tor-vpn over built-in Snowflake or obfs4. Both features are not yet supported by onionmasq. This issue will be mitigated as soon as tpo/core/onionmasq#54 (closed) has been finished, however this crash can still occour in certain internal error cases and needs to be fixed separately in the onionmasq repository.

Manual bridge selection with non-obfuscated bridges can be tested now though. Get these bridges here: https://bridges.torproject.org/bridges/?transport=0