Stop requiring deprecated EditPage::getCheckboxes()

Instead, use getCheckboxesDefinition(), and build our save form
checkboxes from that rather than extracting them from the HTML.

The ability to have non-checkbox fields there is removed, as that was
never intentional and is now impossible.

To avoid transient problems during deployment (old JS code cached in
the user's browser receiving the new format of API responses), the old
property is kept in the API response. We should remove it next week.

Bug: T174613
Bug: T174686
Change-Id: I5bfca5e116fe790302c3b6ac1357e80237fb1ed2