Implemented changing of image captions. Behavior is as follows: If both
the photo's name field is empty, it only changes the photo's caption field.
If the photo's name field is not empty, it will validatethe photo name and then validate the caption.

Changed the regex for caption validation to allow blank captions, in order to
allow captions to be removed.

Fixed some bugs in the backend utilities functions related to captions.
Removed redundant validation for captions in the backend.
Merge branch 'master' of https://github.com/bsanders/softeng