Fixed #23690 - fixed examples of manual rendering of form fields

Documentation for rendering form fields manually is now updated to use fields id_for_label instead of hardcoded values with additional mention of label_tag for alternative generation of complete label tag.
Removed references to the deprecated assertRaisesRegexp method.
Designated Django 1.8 as the next LTS.
Merge pull request #3524 from nmundar/ticket_23690

Fixed #23690 - fixed examples of manual rendering of form fields
Reduce calls to state.render in migrations

`state.render` is actually very long, so this commit easily reduce the
number of times it needs to be called by moving some calls only after
`allowed_to_migrate` was called (to avoid calling render states we won't
need)