Move langauge detection to separate module to be more reusable
Add option to disable vendored file exclusion from file search
Allways show all language stats for search
The choice regarding which forms should or should not trigger a warning
is subjective. I tried to be consistent and not warn about forms that:
- run an action, rather than edit data: search, send an email.
- delete data: a warning about losing data would be confusing
Note that forms on sign-in pages were already ignored (using a selector,
rather than an explicit class on the form element).
Fixes #3698.
