stop broadcast video before starting playback (HTML video element)

[aoakley-roku]

This is a partial fix for issue #50 - it covers the HTML video element implementation but not the MSE one (which doesn't ever seem to have had this kind of code).

I don't understand why the code was originally changed in the way it was, so perhaps this reintroduces some issue that change d7dff31a29bd fixed.

[Murmur]

changes was done to the most recent internal git repository, github repository is soon updated to reflect the current codebase (refactoring jenkins, gitlab etc integrations).