[CLC-1230] Front-end hookup for clear completed tasks, bug fix on the status code

JIRA: https://jira.media.berkeley.edu/jira/browse/CLC-1230

- A few thing i'm not too thrilled with I'm punting on.
  - Button color: Sticking with the default foundation style, be more than willing change it to whatever a designer would prefer. The default makes it hard to distinguish from the top filter toggles, so it'd be preferable if it was very clear that clear completed tasks does something else
  - Not thrilled with the lack of error handling/messaging.