accept multiple images in activity/post.json (fixes #3400, BP from #3203)

(cherry picked from commit 8064b916465136cfdd8056cb23a012e60021145d)
add testcases for activity/post.json with multiple images (refs #3400, BP from #3203)

(cherry picked from commit 502a85ad9abae0416c8e574e6e1c1bc7cb10c6c9)
add support for multiple activity images in activity/*.json template (fixes #3400, BP from #3203)

(cherry picked from commit b1c33881a08af31718adabe54c97713c23d99084)
add helper function for ActivityImage (refs #3203)

(cherry picked from commit 914b96596d77b72d9b1a981d33159d1b4cd82b17)
small change of the name in activity/*.json response (fixes #3203)

(cherry picked from commit c7ce43f00f8e5b1c613673b59038692cdb26c317)