Edited image attributes should override data-parsoid value.

Also rework how the size options are handled; lean on the generic
attribute mechanism for handling original wikitext for height/width
attributes.

Update parsoid-specific image tests; add additional tests with data-parsoid
to also test handling of existing content.

Causes a regression in a test using a manual thumbnail; see follow-on
patch I91d170aa4f6d1fd844d24ccb85b886eb079e3e2a for renewed manual
thumbnail support.

Causes regressions in the round-tripping of image dimension strings, as the
original string (50px vs 50x50px) is no longer remembered and default
serialization policy is bounding box only (50px). To be fixed in a follow-up
patch.

Bug: 48665
Bug: 48924
Bug: 49221
Bug: 56400
Change-Id: If5bef14e27d2d2ee944040dbca3b4c6cdef67c7e