page.edit.js: Preserve content of edit summary on quality change

Currently when you click on wpQuality radiobuttons to change
Page quality (e.g. to Proofread, Verified, etc..)
any edit summary previously entered into the field is removed.
This patch changes that behavior, so edit summary entered by
the user is preserved and prepended with page quality status,
not changed to it.

I decided not to use additional variables, though it may raise some eyebrows.

Change-Id: I11588bc47dd15522d2dec14444c3f8e6b09dea4d
Merge "page.edit.js: Preserve content of edit summary on quality change"