refs #7608 - i18n fixes, tests, use POST for action + only display link if authed

* i18n interpolation wasn't right (was inside _() instead out of outside) and one word was being passed in as a URL parameter, so that's now hardcoded in the string so it can be translated in full.
* Rewrote the strings a bit to improve the English.
* Changed method to POST as it modifies data and switched URL helper to hide links when unsuitable.
Disabled mode provides a worse UI in my opinion.  It shows as plain text (and text alignment is broken, incidentally) with no indication to the user as to why it's disabled.  It also doesn't match our usual UI behaviour.