Update our sqlalchemy schema generation to not include constraints

[rtibbles]

Summary

• Updates our schema generation as it seems to have started adding constraints by default
• This can cause issues with content import and annotation, so we prevent this from being done
• Regenerates schema accordingly

Reviewer guidance

As long as all backend tests pass, this should be fine.

[github-actions]

Build Artifacts

Asset type	Download link
PEX file	kolibri-0.18.0a2.dev0_git.43.g060ade26.pex
Windows Installer (EXE)	kolibri-0.18.0a2.dev0+git.43.g060ade26-windows-setup-unsigned.exe
Debian Package	kolibri_0.18.0a2.dev0+git.43.g060ade26-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.18.0a2.dev0+git.43.g060ade26.dmg
Android Package (APK)	kolibri-0.18.0a2.dev0+git.43.g060ade26-0.1.4-debug.apk
Raspberry Pi Image	kolibri-pi-image-0.18.0a2.dev0+git.43.g060ade26.zip
TAR file	kolibri-0.18.0a2.dev0+git.43.g060ade26.tar.gz
WHL file	kolibri-0.18.0a2.dev0+git.43.g060ade26-py2.py3-none-any.whl