Bug 852185: Hiding contribute form for a specific locale

Locales that have `## hide_form ##` on the first lines will force the contribute page to hide the form.

This use the same idea (and code) of `l10n_utils.dotlang.lang_file_is_active()`

Sorry, I didn't create any test case for this. But I tried with few locale files and everything went well, as expected.