Add respondent count to candidates page

[jrubenoff]

Outcome

Admins should know how many candidates they're inviting to interview.

Structure

When the admin is selecting candidates through a Screendoor filter, show the admin how many responses they've selected.

[jrubenoff]

II was thinking something like this:

As the admin changes the page's settings, the count will change. (Might be nice to animate the numbers to highlight the count?)

[ajb]

This issue was moved to dobtco/scheduler#24