Fixing image tags after form validation error

When editing image tags, the values displayed
are now computed on the fly from the value, if any
that was last entered (existing tags, or perhaps
tags that were entered but submitted along with
an incomplete form).

Closes-bug:  #1270965

Change-Id: Iec9bfae8a0c0da34751ba95274dc58eb7bc09d2e
Merge "Fixing image tags after form validation error"