feat #1617 Adds link in front to download gallery.
doc #1617 Updates documentation of gallery module
feat #1617 Removes unuse methods and refactors request on gallery module.
test #1617 Adds test case in DownloadGalleryViewTest

Adds a test case in DownloadGalleryViewTest to check if zip
file has all images of the gallery.
fix #1617 Refactors get_users method in gallery/models
feat #1617 Adds cache system for the export of a gallery.

All zip files are on a cache folder, gallery-cache, at the
root of the project, removes all zip files if the size of
the cache folder is bigger than the value specified in the
settings file and removes current zip file if it exists and
there isn't new images in the gallery.