Hierarchical view style fixes

[UnniKohonen]

Reasons for creating this PR

Hierarchical view has several minor style errors, this PR fixes them.

Link to relevant issue(s), if any

• Part of Epic: Implement hierarchical view in sidebar using Vue #1521

Description of the changes in this PR

• Fix width of concepts
• Remove unnecessary borders
• Set last class correctly
• Use images instead of unicode characters in buttons

Addresses 3g, 3h and 3c in #1521

Known problems or uncertainties in this PR

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)

[osma]

I tested this briefly and the changes look very good! But there are currently two problems:

1. There is a conflict that needs to be resolved
2. The Cypress Axe test for the concept page fails for some reason

If you can fix these, then this can be merged!

[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 SonarCloud