Make new graphical interface for editing existing
galleries and adding new galleries.
NB The dialog does not yet support rich text in the
image captions, nor does it provide separate fields for
e.g. link, alt text, etc. These are dependent on parsing
the text within the tag, which is yet to be implemented
by Parsoid. For now, these attributes should be
specified in wikitext in the image-specific caption
field.
Bug: T45037
Change-Id: I2b4082e991268241a15b9bbd6d85c94cdc2185f2
