Fix buttons running together on the post setup modal

[MisRob]

Summary

Before	After

References

Fixes #10886

Reviewer guidance

I wasn't able to test it in a regular way due to #10959, so you may run into the same problem too. I needed to temporarily tweak several places in the code responsible for displaying the modal to be able to preview it and make these fixes.

It's possible #10959 is only happening under certain circumstances so you can give it a try by

1. Start a first facility
2. Start a fresh instance of a second facility. In the setup wizard, follow Group learning > Full device > Import all data from an existing learning facility and import data from the first facility.
3. After you're redirected to the Device Channels page, click Continue on the Welcome modal
4. Preview the Select a source modal

Testing checklist

• Contributor has fully tested the PR manually
• If there are any front-end changes, before/after screenshots are included
• Critical user journeys are covered by Gherkin stories
• Critical and brittle code paths are covered by unit tests

PR process

• PR has the correct target branch and milestone
• PR has 'needs review' or 'work-in-progress' label
• If PR is ready for review, a reviewer has been added. (Don't use 'Assignees')
• If this is an important user-facing change, PR or related issue has a 'changelog' label
• If this includes an internal dependency change, a link to the diff is provided

Reviewer checklist

• Automated test coverage is satisfactory
• PR is fully functional
• PR has been tested for accessibility regressions
• External dependency files were updated if necessary (yarn and pip)
• Documentation is updated
• Contributor is in AUTHORS.md

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.0a19.dev0_git.14.g12b2a590.pex
Windows Installer (EXE)	kolibri-0.16.0a19.dev0+git.14.g12b2a590-unsigned.exe
Debian Package	kolibri_0.16.0a19.dev0+git.14.g12b2a590-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0a19.dev0+git.14.g12b2a590-0.3.0.dmg
TAR file	kolibri-0.16.0a19.dev0+git.14.g12b2a590.tar.gz
WHL file	kolibri-0.16.0a19.dev0+git.14.g12b2a590-py2.py3-none-any.whl

[akolson]

This change looks correct to me! Thanks @MisRob

[rtibbles]

Fix manually confirmed.