[WIP] Frontend for time exclusion feature

[sjhuang26]

This is a work in progress. Do not merge this yet. Initial work for the feature where users can click on times in the schedule to exclude them.

Information about which times are excluded is sent to the backend.

Things to do:

• Make the UI look better
• Send data to the backend in a neater format
• What happens when the user clicks on a scheduled event/class? Currently, the time slot behind that event become excluded.