Remove CoreSnackbar from core API.

[rtibbles]

Summary

• Removes CoreSnackbar from the core API.
• Moves it into the GlobalSnackbar component where it is used.
• Keeps them separate for now, as the creation and teardown of the inner component is important for the life cycle of the snackbar.

References

Fixes #12625

Reviewer guidance

Do snackbars still work as intended?

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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

• 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-0.17.0a0.dev0_git.215.gdb893f86.pex
Windows Installer (EXE)	kolibri-0.17.0a0.dev0+git.215.gdb893f86-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0a0.dev0+git.215.gdb893f86-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0a0.dev0+git.215.gdb893f86.dmg
Android Package (APK)	kolibri-0.17.0a0.dev0+git.215.gdb893f86-0.1.4-debug.apk
TAR file	kolibri-0.17.0a0.dev0+git.215.gdb893f86.tar.gz
WHL file	kolibri-0.17.0a0.dev0+git.215.gdb893f86-py2.py3-none-any.whl

[nucleogenesis]

Code changes LGTM - smoke tested locally and was able to find my way to the schnaaaaaackbars