Use relative paths for WebSocket and mopidy.js

[tkem]

See mopidy/mopidy.js#3

Both the path to mopidy.js and the ws path should default to ../mopidy/ instead of the absolute /mopidy/ for use with reverse proxies. Maybe also make these configurable for more advanced settings.

[tkem]

Implementation hint: These could be passed to the app using data-* attributes in the tornado.html template. This would also make the check for isWebExtension() somewhat cleaner.

[tkem]

TBD: How to resolve relative image URIs, such as those generated by mopidy-local-images. Probably this should be left to the proxy server...

[tkem]

In the app's "Add Connection" page, it must also be possible to override the ws path.

[tkem]

It might also be convenient to include our own version of mopidy.js with the Web extension:

• no hassle with reverse proxies
• known feature set, for example Mopidy.when in v0.5.0
• better client caching when used with Tornado's static path versioning (to be verified), or even bundling all JS resources in a single minified file