Fixed login frequency calculation for multiple granularities.

Fixes #465. Turns out, when calculating increment values using lambdas for multiple granularities, we were overwriting the lambda from the first granularity, so it wasn't being calculated independently for each other granularity.

This fixes that and adds the graphs back to the analytics dashboard (all three tabs, "All", "Students", and "Staff").

After merging and pushing this, need to run:

```
bundle exec rake analytics:rebuild_aggregate[CourseLogin]
bundle exec rake analytics:rebuild_aggregate[CourseRoleLogin]
bundle exec rake analytics:rebuild_aggregate[CourseUserLogin]
```