Extract text to translation; fix double-render error

Extract the alert text from SuggestionsController to translation string,
add test for, and fix, latent DoubleRenderError in SuggestionsController

Rename check_mode_of_operation to check_action_allowed, because that's
what it was doing.

Ignoring the guard_action helper I added previously because I think, on
balance, the DRYness is outweighed by the extra work (generate alert and
redirect path on every request) and slightly awkward interface.
Stop using action_allowed_guard; rename guard filters

Stop using action_allowed_guard because, as noted in the previous
commit, it wasn't really making life any easier, and was causing more
work per-request.

Rename Selections controller's guards to reflect what they're actually
doing - checking if voting is allowed, and checking if the chosen
selections can be seen.