Implement total credit counters

[raxod502]

From @PythonNut on January 12, 2018 6:24

Currently, no credit totals for the current schedule are displayed. This is a useful feature and should be brought back.

Plan

• Implement a component that displays course credit totals.
• Implement a container that computes the credit totals from the store, reusing the logic from the original hyperschedule (which should be placed in src/model.js).
• Rejoice and be merry.

Note that this does not require any modifications to the application state management, which should make it relatively straightforward.

Copied from original issue: hyperschedule/hyperschedule#37