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.
Sure, revert the commit if you like, or file it as a new bug introduced by 7608.  I don't think my PR changes this in any significant way.

(Incidentally, the smart proxies page used to have this behaviour before I added the refresh features link in c3d8dd6.)