Fix imgur gifv download link

Adding the downloadUrl to the info.gifv to be passed as downloadurl to the template. This is different than the direct URL as it will actually give the user a file download prompt.
fix imgurgifvUI download link

it was trying to load {{ source }} outside of {{#sources}}{{/sources}} resulting in href="". Added downloadurl taken from info.downloadUrl which will actually prompt the user to download the file instead of just opening another tab/window.
Merge pull request #1684 from markekraus/patch-3

populate imgurgifvUI download link with real url