Fix Welcome Modal for different browser sessions

[ozer550]

Summary

• Fixes the bug with display of Welcome Modal w.r.t multiple browser sessions.
• Move PostSetup Modal to index level.
• Use getter function along with localStorage to decide display of Welcome Modal.

References

closes #11456

ScreenShot

(welcome modal does not get displayed after clearing the DEVICE_WELCOME_MODAL_DISMISSED key from cookies)

Reviewer guidance

1. Sign in and go to the Library page and/or Device > Channels - observe the welcome modal.
2. Import channels
3. Clear all the browser cookies and sign in again, or sign in using a different browser, Welcome Modal is not displayed now!

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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-0.16.0b8.dev0_git.18.g22dfa6f4.pex
Windows Installer (EXE)	kolibri-0.16.0b8.dev0+git.18.g22dfa6f4-unsigned.exe
Debian Package	kolibri_0.16.0b8.dev0+git.18.g22dfa6f4-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.0b8.dev0+git.18.g22dfa6f4-0.3.0.dmg
Android Package (APK)	kolibri-0.16.0b8.dev0+git.18.g22dfa6f4-0.1.0-debug.apk
TAR file	kolibri-0.16.0b8.dev0+git.18.g22dfa6f4.tar.gz
WHL file	kolibri-0.16.0b8.dev0+git.18.g22dfa6f4-py2.py3-none-any.whl

[marcellamaki]

Great work @ozer550!

Code review looks good, and I have tried the following combinations:

• no channels and no cookies set
• channels, but no cookie set
• cookie set, but no channels
  All check out.

@radinamatic - I'm going to go ahead and approve this for testing in the next beta so as to not pull you away from the syncing troubleshooting that you've been working on, but just flagging here for your awareness for testing there.