Truncate the syncqueue table before we add the unique constraint.

[rtibbles]

Summary

• Adds truncation of the syncqueue table to the migration before we add a constraint

References

Fixes #12194

Reviewer guidance

Have I got my SQL right? I only checked the dialect of the default connection, even though technically for SQLite there are multiple databases, but we don't mix and match database types in Kolibri.

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[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.16.2b2.dev0_git.7.g56a41449.pex
Windows Installer (EXE)	kolibri-0.16.2b2.dev0+git.7.g56a41449-windows-setup-unsigned.exe
Debian Package	kolibri_0.16.2b2.dev0+git.7.g56a41449-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.16.2b2.dev0+git.7.g56a41449-0.4.2.dmg
Android Package (APK)	kolibri-0.16.2b2.dev0+git.7.g56a41449-0.1.3-debug.apk
TAR file	kolibri-0.16.2b2.dev0+git.7.g56a41449.tar.gz
WHL file	kolibri-0.16.2b2.dev0+git.7.g56a41449-py2.py3-none-any.whl

[bjester]

LGTM