Fix Add Course button in timetable misalignment

[kaze-droid]

Context

Fix for Issue #3827

Implementation

Added additional css styling for small and medium media breakpoint in Modals.scss and ModulesSelect.scss

Other Information

Before: Modules select tip not visible in Small breakpoint

Before: Modules select downshift component not properly aligned in Medium breakpoint

After for Small & Medium:

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 54.63%. Comparing base (2d4743d) to head (2dc8861). Report is 32 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3829 +/- ## ========================================== + Coverage 53.87% 54.63% +0.75% ========================================== Files 274 274 Lines 6032 6066 +34 Branches 1449 1453 +4 ========================================== + Hits 3250 3314 +64 + Misses 2782 2752 -30 ``` | [Flag](https://app.codecov.io/gh/nusmodifications/nusmods/pull/3829/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nusmodifications) | Coverage Δ | | |---|---|---| | [](https://app.codecov.io/gh/nusmodifications/nusmods/pull/3829/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nusmodifications) | `54.63% <ø> (?)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=nusmodifications#carryforward-flags-in-the-pull-request-comment) to find out more.

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