feat: highlight buttons when selected using keyboard tabbing

[ghost]

This PR highlights buttons when selected using the keyboard. It does so by making all attributes on the CSS pseudo-class .btn(-primary|-danger):hover also apply to .btn(-primary|-danger):focus.
In essence, the UX from hovering over a button now translates directly over to TAB-ing to a button.

This PR closes #1716.

Checklist

Before submitting the PR

• Read the CONTRIBUTING document before submitting your PR.
• If the PR is related to an issue or fix one, don't forget to indicate it.
• [n/a] Indicate [wip] in the title of the PR it is is not final yet. Remove [wip] when ready. Otherwise the PR will be considered complete and rejected if it's not working.

General checks

• Make sure that the change you propose is the smallest possible.
• The name of the PR should follow the conventional commits guideline that the project follows.

Front-end changes

• If you change the UI, make sure to ask repositories administrators first about your changes by pinging @djaiss or @asbiin in this PR.
• [n/a] Screenshots are included if the PR changes the UI.
• [n/a] Front-end tests have been written with Cypress.

Other tasks

• CHANGELOG entry added, if necessary, under UNRELEASED.
• CONTRIBUTORS entry added, if necessary.
• [n/a] If it's relevant and worth mentioning, create a changelog entry for this change. The changelog entry will appear inside the UI for all users to see. To know if your change is worth the creation of a changelog entry, read the documentation.

[ghost]

@djaiss This is like the most minimal change one can make, but it should do the job.
In my instance, the weird click-hold glitch also described in #1716 disappeared as well, but this would warrant more testing if it's really gone.

[djaiss]

@TheZenti excellent work! Thanks so much for your contribution!!!

[github-actions]

This pull request has been automatically locked since there
has not been any recent activity after it was closed.
Please open a new issue for related bugs.