Add more error handling for facility user merging and facility deletion

[rtibbles]

Summary

• Moves IsSelf permissions check into tasks.py as it is specific to the arguments for the mergeuser task
• Updates it to check either the local user id or the remote user id, so after merge, the user is still able to check the task.
• Handles an edge case where we were trying to check if the user was an admin for a facility that no longer existed
• Tries to prevent more weirdness by adding an explicit signal to clear the request.user cache when a user is deleted
• Calls this signal explicitly after calling delete_facility in the mergeuser task because the delete_facility function disables post_delete signals.
• Updates token generation for merge user flow to use only the user_id, so we can set this when first creating the task
• Updated handling for 403s to assume success, as it generally indicates that the user has been deleted and thus successfully merged
• Opportunistically update the global learn inClasses boolean when we fetch class data from the backend.

References

Attempts to fix #12486

Reviewer guidance

While this only seems to happen on Android, I think it's more to do with unhandled errors arising from the process taking longer on Android, so hitting certain coincidences on Android that we might not see elsewhere.

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Reviewer checklist

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Build Artifacts

Asset type	Download link
PEX file	kolibri-0.17.0b3.dev0_git.67.gfc132c22.pex
Windows Installer (EXE)	kolibri-0.17.0b3.dev0+git.67.gfc132c22-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0b3.dev0+git.67.gfc132c22-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0b3.dev0+git.67.gfc132c22.dmg
Android Package (APK)	kolibri-0.17.0b3.dev0+git.67.gfc132c22-0.1.4-debug.apk
TAR file	kolibri-0.17.0b3.dev0+git.67.gfc132c22.tar.gz
WHL file	kolibri-0.17.0b3.dev0+git.67.gfc132c22-py2.py3-none-any.whl