Tweak spacing in responsive layout files for mobile

[tnir]

Summary

Tweaks many spacing in responsive layout files mainly for mobile to reduce unexpected horizontal drifts on mobile screens.

This PR also helps developer experience by replacing home-made CSS selectors with Bootstrap 4's utilities.

Ticket Link

This change is a groundwork for Hugo layouts and not relevant to any specific feature(s).

[mattermost-build]

Hello @tnir,

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[tnir]

@cwarnermm Thoughts?

[github-actions[bot]]

Newest code from cwarnermm has been published to preview environment for Git SHA b8dcfd65ed4a93ad12ce92b8c4a2e721cf58a8e8

[github-actions[bot]]

Newest code from cwarnermm has been published to preview environment for Git SHA b8dcfd65ed4a93ad12ce92b8c4a2e721cf58a8e8

[cwarnermm]

update-branch

[github-actions[bot]]

Newest code from cwarnermm has been published to preview environment for Git SHA b8dcfd65ed4a93ad12ce92b8c4a2e721cf58a8e8

[tnir]

update-branch @cwarnermm

[cwarnermm]

@emdecr - Are you open to taking a peek at this dev docs PR, please? I'd love to get your input on the proposed changes.

[emdecr]

@cwarnermm Sure thing - will circle back within the next day or so.

[tnir]

Thanks @cwarnermm. This change carried me a nice mobile experience on dev guide indeed 🎉