Show alt-text in export dialog and for lightbox image

Pass alt parameter from mmv.bootstrap.js to mmv.js and
set it as a parameter on the displayed lightbox image.
Include the alt text in the embed text.

Bug: T66519
Bug: T75923
Change-Id: I29503eb582ac2bc8cf89f737a3bcb787b660d918
Merge "Show alt-text in export dialog and for lightbox image"