Warn users who are planning modules with lecture clashes

Is your feature request related to a problem? Please describe.

For most cases, students are not allowed to take two modules with clashing lecture slots by NUS. When they are planning, having this feature can help students realise that their lecture slots actually clash.

Describe the solution you'd like

Currently, NUSMods display a warning if two modules share the final exam timing clash. It would be nice to have something similar shown if two modules have clashes in lecture timing (especially if that slot is the only slot that both modules have)

Describe alternatives you've considered

It may visually help users realise that the slots clash if that particular slot is decolourised (using a special colour to bring attention to that particular slot)

Additional context

I'm not sure if NUS actually allows students to take modules with clashing lecture slots concurrently. Currently, a possible example is taking CS3243 and CS2107 in AY19/20 Sem 2:

Both these modules only have 1 lecture slot and they clash.

And as described above, a possible decolourising of the slot may look something like this:

Another thing which can be of concern is that it may be visually very distracting in the early phase of planning (just when the modules are being added) to see all the warnings if this feature is implemented

Start by locating the existing final-exam clash warning in the planner UI and trace how lecture slots are represented and compared. Clarify the intended warning behavior and visual treatment for lecture clashes, including when warnings should appear, then verify the result with the CS3243 and CS2107 example described in the issue.