EditPage: Refactor getCheckboxes() to allow changing the format

EditPage: Refactor getCheckboxes() to allow changing the format

getCheckboxes() directly generated the HTML for the
"This is a minor edit" and "Watch this page" checkboxes,
and allowed extensions to add more HTML checkboxes (and
modify existing ones) using the 'EditPageBeforeEditChecks'
hook. This prevents us from ever changing the format of
the HTML (e.g. to use OOUI checkboxes).

Introduce new method getCheckboxesDefinition(), which
generates the checkboxes in a machine-readable format,
with a new hook 'EditPageGetCheckboxesDefinition'.
Rewrite getCheckboxes() in terms of that. The old hook
'EditPageBeforeEditChecks' is now deprecated.

Change-Id: I3dbe973dcac6cba0c3a1ac5d983cafcfb49d833c