Rework source validation

[meeb]

Currently the add a source flow is tightly restricted and this should be relaxed to allow freeform URLs to be entered which are properly parsed to detect a valid source type. This is the first step in resolving additional YouTube URL formats (e.g. #30) and for future support of other sources that are added to youtube-dl.

The current "add a button to add a source type" UI format isn't flexible enough to add a button per source. There's also more YouTube URL formats than first planned for. Adding a freeform URL box would resolve this, at the expense of writing moderately more complicated URL parsers.