Add download options for videos

[Maniload]

Although you can already download videos from the client through the HTML5 player or by using the embeddable link, it may be nice to also have dedicated options to do so.

There are currently two ideas for this:

• Download button in the theme context menu.
• Bulk download options on various pages (needs server endpoint that bundles multiple videos into an archive).

[Uhhu738]

You can just right-click and select "save as" on the video file or open in a new tab, but it would be useful for people that don't have that option.