Upload CSV file for cohorting students

TNL-735

@andy-armstrong You can start reviewing the front-end changes and tests for this PR. I'll send you an email with some information about the sandbox.

At this point, you can also edit files created in Excel, unless if they have unicode characters in them.
@talbs My only other thought here is maybe we can combine these styles somewhat. For example:
```
.divider {
    border-style: solid;
    border-color: $gray-l4;

    &.lv0 {
        border-top-width: ($baseline/5);
    }

    &.lv1 {
        border-top-width: ($baseline/10);
    }

    &.lv2 {
        border-top-width: ($baseline/20);
    }
}
```
Or something to that effect. Not to say it's not good now, just trying to think of ways to manage it easier?