Preliminary refactor of video support in stream

Shift support for reading video formats into Gallery subclasses, rather
than using TemplateProcessor subclasses.

This allows a more uniform interface (rather than having separate gallery/
video modes, everything goes through galleries), and gallery subclasses
can also be used in other contexts.
Drop explicit streams from most video abbreviations

These can work perfectly well using the default -enroll stream now, and can
still be wrapped if e.g. concurrent processing of multiple videos is desired.
Merge pull request #294 from biometrics/stream_galleries

Refactor video handling in stream