[repo config] cleanup read_instance_schema / bootstrap_schema / creating mess

fix #726167: i18ninstance use data in bootstrab_cubes instead of the database one

[cleanup] remove 3.7 deprecation warning

[form controller] clear caches in case some attribute participating to the rest path has been modified, avoid redirection to a no more existant page (fix #753567)

i18nupdate

[form] fix validation error handling * type when possible ValidationError raised by the edit controller * don't rely on repository to check required field * turn css class used for field error message to errorMsg to avoid confusion with the error class added to input * fix css when errors are added by the form renderer * fix form renderer to add the error message

[captcha] handle captcha validation properly in the captcha widget also, avoid error if pil isn't installed (only a recommendation)

[testlib] __errorurl may be a relative path, check we're actually removing base url first

[i18ncube] clear_rtag_objects imported when generating cube's .po file

[maintainance] don't crash if we've no in-memory repository