#2003 - Add admin/movies screen.

Added admin/movies screen analogous to the admin/graphics screen so the user can:
- see how FFmpeg is configured (path and version, similar to toolkits in admin/graphics)
- get some instructions on how to install FFmpeg if not found
- change the movie_allow_uploads setting
- ask Gallery to rebuild their movie thumbs

Specifics:
- admin_movies, admin_movies.html (new) - new Movies admin screen
- ffmpeg.png (new) - logo for admin screen
- movie::get_ffmpeg_version (new) - return version number and date of FFmpeg
- form_uploadify.html - change admin message if movie uploads are disabled
- gallery_event::admin_menu - added Movies link to Settings