Contribute: Fix RecursionError for xblock translations

[OmarIthawi]

Contribute both #953 and #956

This exception below is raised because there's an attempt to make the ModuleI18nService fallback to itself due to the use of the magical ModuleI18nService.__getattr__ which returns django.utils.translation._fallback when asked for ModuleI18nService._fallback. RED-2263.

This creates a infinite recursion that needs to break.

File "xblockutils/templatetags/i18n.py", line 41, in merge_translation
    translation.merge(i18n_service)
...
File "python3.5/gettext.py", line 169, in add_fallback
    self._fallback.add_fallback(fallback)
RecursionError: maximum recursion depth exceeded"

Tests

I wasn't able to reproduce the issue via scripts or tests. The issue has extended beyond the regular bug timebox of 4 hours.