Fix "skip content"-buttons on small screen sizes

[susnux]

• Resolves: [BITV] 9.1.4.10/3.1 - At a browser window width of 320 CSS pixels (or at a browser window width of 1280 CSS pixels and 400% zoom magnification), the jump links at the top of the page are overlaid and displayed cut off. (8) #36963

Summary

If the screen size is too small (this case 320px) the buttons overlapping each other, this is fixed by displaying them using flex box.

Screenshots

before	after

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Screenshots before/after for front-end changes
• Backports requested where applicable (ex: critical bugfixes)

[susnux]

/backport to stable26

[susnux]

/backport to stable25

[backportbot-nextcloud]

The backport to stable25 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable25
git pull origin/stable25

# Create the new backport branch
git checkout -b fix/foo-stable25

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts. Resolve them.
git cherry-pick abc123

# Push the cherry pick commit to the remote repository and open a pull request
git push origin fix/foo-stable25

More info at https://docs.nextcloud.com/server/latest/developer_manual/getting_started/development_process.html#manual-backport

[backportbot-nextcloud]

The backport to stable26 failed. Please do this backport manually.

# Switch to the target branch and update it
git checkout stable26
git pull origin/stable26

# Create the new backport branch
git checkout -b fix/foo-stable26

# Cherry pick the change from the commit sha1 of the change against the default branch
# This might cause conflicts. Resolve them.
git cherry-pick abc123

# Push the cherry pick commit to the remote repository and open a pull request
git push origin fix/foo-stable26

More info at https://docs.nextcloud.com/server/latest/developer_manual/getting_started/development_process.html#manual-backport