switch to Django's i18n

[escattone]

In preparation for Wagtail, switch from our current custom i18n code to Django's i18n code, with the addition of a custom "shim" that ensures uppercase country codes within locales that include them (for example, en-US instead of en-us).

[emilghittasv]

The testing activities for the Django i18n work are finalized. The following activities were covered:

• Verified the redirect from lower case country codes to uppercase (example: en-us will redirect automatically to en-US. Tested using different locales available & mentioned inside https://github.com/mozilla/kitsune/blob/e4b59db2c540188f4e90cabb680025b18165aa3a/kitsune/settings.py)
• Verified that the explicit fallbacks are still in place (example: sc -> it. Tested using different locale fallbacks mentioned inside https://github.com/mozilla/kitsune/blob/e4b59db2c540188f4e90cabb680025b18165aa3a/kitsune/settings.py)
• Verified that SUMO page locale can be successfully changed via the footer section.
• Verified that profile preferences (in terms of preferred language) are successfully applied and SUMO switches to the chosen language successfully.
• Verified that the Accept-Language header value is successfully respected and SUMO fetches the given locale (also tested fallback cases. Example: tested that setting sc won't automatically fallback to it. It will instead fallback to en-US if no additional languages are given to the header. If we have additional languages which are supported it will successfully fallback to those).
• Verified that Trying to access the "View Profile" and "My Questions" pages while signed in with accounts that contain special characters (+) inside the username leads to "Page not Found" #1496 is no longer reproducible & re-triggered the automated tests to see if we have failures. We are back to green now.
• Performed an extended round of exploratory testing. Navigated the SUMO website while switching between multiple locales, performed translations, accessed translated articles,etc.

Since no new issues were encountered I'm moving this ticket inside the "Release" column.

[escattone]

Released!