Disable Term Selector options for currently unavailable terms

### Description

Currently, the term selector shows all terms and user can select any term. We want to change this so that only terms that Notangles has course data on is selectable.

### Requirements

- term selector has all options unchanged
- terms that do not have data yet is not selectable

Find the term selector entry point and trace how available course data is represented. Keep every term visible, make terms without course data unselectable, and verify that terms with data remain selectable.

Written by the indexing model from the issue text.