Update Last Visit Date graph on At-Risk Learner dashboard

[crathbun428]

User Story
As a Course Delivery Team member, I want to update the default time grain on the Last Visit Date, so that its easier to read.

Acceptance Criteria
Given I am listed as course staff on one or more courses
When I view the at-risk learner dashboard for a course I'm listed as staff
Then I see the Last Visit Date graph is updated so that each bar represents the number of learners that last visited the course on each distinct date (not datetime as it currently shows)