Add notification for downloaded content completion

[LianaHarris360]

Summary

This pull request involves the addition of a snackbar notification with the message “Download complete” to inform learners about resource download completion.

Once a resource is selected to be downloaded, a notification with the message “Download requested” and the option to “GO TO DOWNLOADS” appears. Now, on the remote content page, the learner is also notified when the download is finished.

topicsContent.mp4

References

Closes #10576

Reviewer guidance

Ensure that only after adding a new download and the content download is completed, the completed download snack-bar notification is displayed.

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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

• 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-.pex
Windows Installer (EXE)	kolibri-0.17.0a0.dev0+git.26.gf9b4633e-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0a0.dev0+git.26.gf9b4633e-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0a0.dev0+git.26.gf9b4633e-0.4.2.dmg
Android Package (APK)	kolibri-0.17.0a0.dev0+git.26.gf9b4633e-0.1.3-debug.apk
TAR file	kolibri-0.17.0a0.dev0+git.26.gf9b4633e.tar.gz
WHL file	kolibri-0.17.0a0.dev0+git.26.gf9b4633e-py2.py3-none-any.whl

[MisRob]

I read through code and it's looking good to me. Thank you @LianaHarris360