[Forms] Redirect on successful save

[driusan]

This updates the NDB_Form class to respond with a 303 See Other after a successful page and redirect to the same page.

This (in addition to being a better practice) fixes problems where the page is rendered before changes that depend on the updated values are saved to the database.

In particular, when changing the language on the my_preferences page, it will now immediately take effect rather than only doing after after the next page load.

Blocked by #10031

[CamilleBeau]

@driusan the PR needs an update for test suite

[driusan]

@CamilleBeau rebased