web ui stream fixes

[frecuencialibre]

This pr gets us part of the way to fixing #16 out of the box:

• ensure the instance can find itself at EXTERNAL_HOSTNAME. this avoids the error message in the stream settings you can see in the screenshot in the issue above.
• set the EXTERNAL_HOSTNAME as the default instead of localhost

at this point i think that providing an out-of-the box solution to #16 would require resolving #20. alternatively we could just add instructions for the Settings > Streams post-install step to the readme.

[frecuencialibre]

this PR would also close #11

[ned-kelly]

Approved, however we need to test that the "generalHost" (in the setup wizard) still works ok.

fe9faa8