Add :allow_destroy option to has_many forms

This option will add a boolean field "Remove"
to the end of each has_many fieldset that is
an existing record. This provides a workaround
for issue #1990, and allows for cleaner form
coding.
Add i18n for has_many_remove label

Conflicts:

	lib/active_admin/locales/cs.yml
	lib/active_admin/locales/de-CH.yml
	lib/active_admin/locales/en-GB.yml
	lib/active_admin/locales/pt-PT.yml