Render check boxes from EditPage

EditPage has a lovely getCheckboxes() function which includes the
minor and watch checkboxes as rendered by MW core, as well as any
checkboxes extensions like FlaggedRevs might have added. Output
these in the API, render them, and send their values back.

ApiVisualEditor.php:
* Build a fake EditPage, get its checkboxes, and return them

ApiVisualEditorEdit.php:
* Pass through posted request data to ApiEdit, which passes it
  through to EditPage thanks to Idab5b524b0e3 in core

ve.init.mw.ViewPageTarget.js:
* Remove minor and watch checkboxes from the save dialog template
  and replace them with a generic checkbox container
* Have getSaveOptions() pull the state of all checkboxes in
** Special-case minor and watch, and pass the rest straight through
** Move normalization from true/false to presence/absence here, from
   ve.init.mw.Target.prototype.save(), because here we know which ones
   are checkboxes and we don't know that in save() without
   special-casing
* Remove getSaveDialogHtml(), we don't need to hide checkboxes based on
  rights anymore because in that case the API just won't send them to us.
** Moved logic for checking the watch checkbox down to where the same
   logic for the minor checkbox already is
* Unwrap getSaveDialogHtml() in setupSaveDialog()
* Access minor and watch by their new IDs throughout

ve.init.mw.Target.js:
* Get and store checkboxes from the API
* Pass all keys straight through to the API

Bug: 49699
Change-Id: I09d02a42b05146bc9b7080ab38338ae869bf15e3