Allow uploading of custom scene covers

[WithoutPants]

Adds a "Cover Image" section to the Edit Scene page. This is an expandable section that is collapsed by default, as follows:

Expanding this shows the current cover image, with a file selector to select a new cover image:

Like the other image selection pages, Ctrl-V with an image in the clipboard will paste the image directly.

Note that saving the scene after updating the cover image does not update the cover image in the video player until the page is reloaded.

Should address #35

[bnkai]

Looks ok.
Tested the addition through file browse, paste.
Works ok in the scenes grid,list and the player.
It replaces the original screenshot, thumbnail so no problem with the clean task also.