fix: improve alerts & reports modal on small devices

[diegomedina248]

SUMMARY

The alerts & report modal content is divided into columns, but the content is neither responsive nor fluid, so it doesn't play well on small screen sizes.

This PR introduces small tweaks to ensure the content from one column does not overflow to the next.
The timezone selector is reduced when needed & the text inside it is truncated (on small screens).

A better way to approach this would require using media queries, but the viewport meta tag is not defined and thus the media queries based on width do not work.
It's a larger discussion to be had the path to introduce responsive design.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Before:

After:

TESTING INSTRUCTIONS

1. Open alerts & reports menu item
2. Create an alert or report
3. With the modal up, make your browser narrow

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #19294 (a932cdd) into master (9ae51f7) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master   #19294   +/-   ##
=======================================
  Coverage   66.63%   66.64%           
=======================================
  Files        1670     1670           
  Lines       64570    64590   +20     
  Branches     6504     6507    +3     
=======================================
+ Hits        43028    43047   +19     
- Misses      19858    19860    +2     
+ Partials     1684     1683    -1     

Flag	Coverage Δ
javascript	51.39% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...frontend/src/views/CRUD/alert/AlertReportModal.tsx	52.38% <ø> (ø)
...frontend/src/components/TimezoneSelector/index.tsx	96.96% <100.00%> (+0.09%)	⬆️
superset-frontend/src/views/components/SubMenu.tsx	87.50% <0.00%> (-1.87%)	⬇️
superset-frontend/src/SqlLab/actions/sqlLab.js	60.73% <0.00%> (ø)
superset-frontend/src/views/components/Menu.tsx	60.00% <0.00%> (ø)
...ontrols/DndColumnSelectControl/DndColumnSelect.tsx	38.09% <0.00%> (ø)
...perset-frontend/src/views/CRUD/welcome/Welcome.tsx	75.00% <0.00%> (+0.51%)	⬆️
superset-frontend/src/views/CRUD/utils.tsx	65.57% <0.00%> (+0.61%)	⬆️
...tend/src/views/CRUD/data/database/DatabaseList.tsx	67.39% <0.00%> (+1.91%)	⬆️
...perset-frontend/src/views/components/MenuRight.tsx	67.69% <0.00%> (+1.97%)	⬆️

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