Create BridgeDB config setting for which Pluggable Transports are supported

Currently, there is a list of the "currently supported Pluggable Transports" in bridgedb.strings. This module attribute can be used to change which PTs are listed in the dropdown menu on https://bridges.torproject.org/options, as well as in the list sent out in email responses. Changing this will easily change what both UIs display, however, there still are some sections of BridgeDB's code where the supported PTs are hardcoded.

We need to:

1. Identify where and why PT method names are hardcoded.
2. Create a config setting in bridgedb.conf to set this list globally (in the UI, and in code logic).
3. Get rid of all the hardcoded PT method names in (1) and replace them with the thing from (2).