When entering postcode, Save button ignores postcode

[aspiers]

In both /user/constituency/edit and /user/edit, the Save button does not trigger a constituency search from postcode, so effectively the postcode field is ignored. This is bad because users will expect to be able to just enter postcode and hit Save. Requiring them to hit Search first is unintuitive.

[aspiers]

One side effect of this is that one test user thought that invalid postcodes are ignored. Although in fact all postcodes are ignored unless Search or the Enter key are pressed.

[baob]

See also #636