add basic OGV video support

 * work on https://github.com/sebsauvage/MinigalNano/issues/46
 * lightbox not supported yet, videos are just linked from the gallery page

This can easily be extended to other video formats, just add filetypes to the `Other filetypes` section, and symlink their thumbnail (eg. `filetype_MKV.png`) to to `filetype_OGV.png`

It would be cool to have videos rendered in the lightbox, but this needs more work and there are current plans to change the lightbox implementation, so I just added basic functionality.