Ability to add custom "blocks" in timetable.

[Zxun2]

Considering https://github.com/nusmodifications/nusmods/issues/3421 and #3404, i wonder if it is possible to add custom "blocks" in the timetable. This can allow us to keep track of recurring project meetups, TA duties and club/societies commitments.

A possible way to do this is similar to the current implementation made in the planner tab, where users can create a new module (stored locally). Another improvement to this feature would be to add custom tags and color to the "blocks".

[jloh02]

Similar issue is being tracked in #2040 and proposed to be resolved in PR #3373