Ensure login button is not broken into two rows

[edlerd]

Done

• Changed login screen layout, so it won't break the "Create a new certificate" button is not split into more than one row on medium-sized screens

Fixes WD-9657

QA

1. Run the LXD-UI:
   • On the demo server via the link posted by @webteam-app below. This is only available for PRs created by collaborators of the repo. Ask @mas-who or @edlerd for access.
   • With a local copy of this branch, run as described here.
2. Perform the following QA steps:
   • test the login screen on various screen widths.

[webteam-app]

Demo starting at https://lxd-ui-711.demos.haus

[mas-who]

QA looks good, just one small query regarding the new styles.