[i18n] Wrap easy strings in gettext function

[driusan]

This wraps the long hanging fruit of user-facing strings on the PHP side of LORIS in the gettext function (using the _ alias). It does not comprehensively do every string, because:

1. Some strings have variables embedded. A lookup into the gettext would not translate these
2. modules//ajax/ scripts are ignored, because they bypass the index.php/router/etc where the the domain is likely to be set by LORIS. The work of fixing them is assumed to be less than the work of updating them to proper endpoints.
3. Strings that weren't obvious if they would break anything are ignored (ie. keys in an array)
4. Probably some other that I'm forgetting

Nonetheless, it's a first pass at marking up our code with translatable strings.