Minor fixes

1. Automatically adjust the threshold for a valid exam schedule, based on the term (Spring vs the rest)
1. Render the login modals on every page (because they appear on the nav bar, which is on every page). For example, before this PR they didn't work on the unsubscribe page.
You should move this `include` line outside of the `{% block templates %}` block. If there are any child templates that override `{% block templates %}` (and lots do), then this file will not be includes unless the child block calls `super()`.