Built-in bridge refactoring.

Store the built-in bridges in OnionShare settings as a dict, and try writing those into the torrc if present.

If they aren't present in OnionShare settings, use the hardcoded templates instead.

Fetch the latest built-in bridges over Tor, once connected to Tor.

If we can't fetch the bridges over Tor, fall back to Meek (domain-fronting) and try again.

Then write those as the dict to the OnionShare settings, to take precedence next time.

cli/onionshare_cli/settings.py

@@ -110,6 +110,7 @@ class Settings(object):
             "bridges_builtin_pt": "obfs4",  # "obfs4", "meek-azure", or "snowflake"
             "bridges_moat": "",
             "bridges_custom": "",
+            "bridges_builtin": {},
             "persistent_tabs": [],
             "locale": None,  # this gets defined in fill_in_defaults()
             "theme": 0,