StashEdit: Refactor and simplify edit.stash.js

StashEdit: Refactor and simplify edit.stash.js

* Reduce call sites for stashEdit() to only inside checkStash().

* Update onFormLoaded() to not call stashEdit() directly but instead
  call checkStash(). This avoids hardcoding the priority and
  simplifies code by ensuring isTextChanged() and isSummaryChanged()
  are always checked before stashEdit() is called.

* Move texthash and priority handling to stashEdit().

* Don't clear lastTextHash if text hasn't changed.

* Remove check for 'origSummary' in onTextFocus. Have stash rate
  always be 3s for the textarea, and always 1s for the summary.

Change-Id: I3f049df6ee2e1c218e8b3c1c397ff0bc50e58c8e