Restrict ListImageElements to authenticated users

Part of #720 (closed) (some frontend work also required)

Note that ListImageElements already had corpus rights filters, so no element information was leaked at all.

changed the description

requested review from @babadie

added 4 commits

• 0ed519ea...b948554f - 3 commits from branch master
• 830fa421 - Restrict ListImageElements to authenticated users

Compare with previous version

mentioned in commit 30fb3a2e

merged