Allow selection of multiple categories in the video uploader

[c510e2d0]

Describe the problem to be solved

The issue is that there are videos which have multiple different categories that others would not wish to see. Being able to only select one category to upload a video as doesn't give the full picture of what the video is.

Describe the solution you would like

This would be an option the server administrator can enable which replaces "Category" with "Categories" in the video uploader. Categories would use a check-box list where you can select multiple categories at once.