Create logs for when users get a badge

[harmitgoswami]

As part of our initiative to increase user engagement using data, we want to know if receiving a badge encourages users to keep engaging more with Pontoon, whether it be through submitting more translations, logging in more frequently, etc.

Currently there is no way to check when a user received a badge, since badge information is not explicitly stored in the database. Thus, we'll need to create/modify a log model to now begin tracking when badges are awarded to a user.

[mathjazz]

We might want to block that on #3443, because ActionLog validation is getting hairy.

Currently there is no way to check when a user received a badge, since badge information is not explicitly stored in the database.

As part of #3442, we'll be sending notifications to users when they are awarded a badge. That should be a good enough workaround until #3443 is fixed.