Add command to flush sync session data

[bjester]

Summary

When sync sessions do not complete properly, they can leave lots of data in the Morango buffers. This is a command that will easily clean those tables.

Reviewer guidance

Run a few syncs, cancelling them after some has transferred, then run this command.

References

Helps: #7270

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Contributor Checklist

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

Testing:

• 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

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

[codecov]

Codecov Report

Merging #7313 into release-v0.14.x will decrease coverage by 0.31%.
The diff coverage is 10.00%.

Impacted Files	Coverage Δ
...ri/core/auth/management/commands/deletefacility.py	0.00% <0.00%> (ø)
...core/auth/management/commands/flushsyncsessions.py	0.00% <0.00%> (ø)
kolibri/core/auth/management/utils.py	40.55% <17.77%> (-7.60%)	⬇️
...libri/plugins/user/assets/src/views/AuthSelect.vue	33.33% <0.00%> (-66.67%)	⬇️
...olibri/plugins/user/assets/src/views/UserIndex.vue	21.05% <0.00%> (-50.83%)	⬇️
kolibri/plugins/user/assets/src/views/AuthBase.vue	28.57% <0.00%> (-47.11%)	⬇️
...ibri/core/assets/src/state/modules/core/getters.js	63.15% <0.00%> (-31.58%)	⬇️
kolibri/core/assets/src/state/modules/session.js	86.20% <0.00%> (-13.80%)	⬇️
kolibri/utils/logger.py	82.50% <0.00%> (-12.50%)	⬇️
...core/assets/src/views/CoreBase/ScrollingHeader.vue	76.08% <0.00%> (-8.92%)	⬇️
... and 26 more

[lyw07]

Tested locally, and looks good to me!