Visual layout improvements especially for landing page

[osma]

Reasons for creating this PR

The landing page didn't follow the visual spec very well and there were issues with font sizes, with some users seeing absurdly large text. This PR attempts to fix the font size inconsistency issue by changing all font-size declarations to use rem units. There are many visual improvements to the landing page layout as well as some adjustments that also affect other page types.

Link to relevant issue(s), if any

• part of Epic: Landing Page #1480 (mostly)
• closes Avoid specifying font sizes in px #1320

Description of the changes in this PR

• always specify font sizes in rem (except for the :root element, where em is better)
• adjust landing page layout to better follow visual spec
• make landing page layout responsive & columns equal height
• adjust footer layout to better follow visual spec, at least on landing page
• set landing page default headline to "Vocabularies" (can be overridden in config.ttl)
• many small adjustments to font sizes, colors etc., also affecting other page types

Known problems or uncertainties in this PR

• I hope I didn't break anything too badly when changing CSS rules

Checklist

• phpUnit tests pass locally with my changes
• I have added tests that show that the new code works, or tests are not relevant for this PR (e.g. only HTML/CSS changes)
• The PR doesn't reduce accessibility of the front-end code (e.g. tab focus, scaling to different resolutions, use of .sr-only class, color contrast)
• The PR doesn't introduce unintended code changes (e.g. empty lines or useless reindentation)

[UnniKohonen]

This looks much better than it did before but there are some things I noticed that should still be fixed:

• The main sans-serif font is not set correctly
• To me the Skosmos logo looks a little big on the landing in comparison to the new sizing
• Vocab titles on vocab home page are no longer centered vertically

I also commented some minor code chages

[UnniKohonen]

A font awesome arrow icon could be added here.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[osma]

Thanks for the comments @UnniKohonen! I think I've now implemented all the suggestions, except the horrible way of constructing links to vocabularies, which I think is a separate problem a bit out of scope for this PR, but we have issue #1565 to remind us about it.

[UnniKohonen]

LGTM!