Enable responsive font sizes

[hardyoyo]

• adds enable-responsive-font-sizes toggle to the main bootstrap
  variables scss file
• docs available

References

• discussed on DSpace slack, tested by myself and @mspalti

Description

This PR adds the enable-responsive-font-sizes toggle to the main bootstrap variables scss file, and toggles it on.

Instructions for Reviewers

You'll need a phone to really test this change, compare the home page type with and without this toggle turned on. With it on, the home page content better fits the phone's viewport, and is thus more usable.

List of changes in this PR:

• adds the enable-responsive-font-sizes toggle to the main bootstrap variables scss file, and toggles it on

Include guidance for how to test or review your PR. This may include: steps to reproduce a bug, screenshots or description of a new feature, or reasons behind specific changes.

As mentioned above, you'll need a phone to test this, or your can use your browser's developer tools, to adjust the viewport to match a phone screen.

Checklist

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[tdonohue]

👍 Thanks @hardyoyo ! I've tested this using the mobile device toolbar in Chrome (which attempts to mimic a mobile device). It definitely looks a ton better with this setting than without it & I do see that the fonts are scaled more appropriately. Also tested with both 'dspace' and 'custom' themes and it works well for both.

[tdonohue]

Merging immediately as this was already previously tested by @mspalti who gave positive feedback via Slack.