extract: Fix extraction for es6/jsx files

[spencerhance]

Django makemessages / GNU gettext do not currently support es6's template strings anywhere in a jsx file (not just in a gettext() call). Since it doesn't appear that gettext has any plans to add this support, we should really find a workaround or another library.

Bug reports:

• https://code.djangoproject.com/ticket/29175
• https://savannah.gnu.org/bugs/?50920

Having backticks in the file causes xgettext's parsing to break, printing a few warnings in the log about unterminated strings or regexes. Additionally, it will complete the parsing but miss many gettext strings, leading to incomplete .po files.

Jira ticket:
https://openedx.atlassian.net/browse/LEARNER-6038