Add an alternative to the zip file for downloading images without addition space on the disk based on https://github.com/ccvca/js-multi-file-download

Works on chromium based desktop browsers (~70% of the users).

Incompatible browsers will see this message:

Requires only minimal changes in the themes.
ccvca/theme-flow@33f4bba

Marked as experimental, as window.showDirectoryPicker() is experimental.

• What does the PR change?
  This will not require addition space a zip-file, as the files are directly downloaded.
• Does it introduce a breaking change for existing users?
  Add new option album-download with the values js and zip and moved album-zip-files to deprecated.

PS: As I don't have much experience with JavaScript, this PR might need some iterations