Restore preseeded database behaviour

[rtibbles]

Summary

• Our sqlite foreign key integrity checks didn't first check if the database file existed
• So when the connection was opened, the file was created
• This was preventing the preseeded database setup from happening

References

Helps to fix #227

Reviewer guidance

Ensure when running kolibri in a fresh env, that database migrations don't happen, and instead it goes straight to setup.

Should also reduce loading times considerably for the Android app.

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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-.pex
Windows Installer (EXE)	kolibri-0.17.0b3.dev0+git.41.g95d28043-windows-setup-unsigned.exe
Debian Package	kolibri_0.17.0b3.dev0+git.41.g95d28043-0ubuntu1_all.deb
Mac Installer (DMG)	kolibri-0.17.0b3.dev0+git.41.g95d28043.dmg
Android Package (APK)	kolibri-0.17.0b3.dev0+git.41.g95d28043-0.1.3-debug.apk
TAR file	kolibri-0.17.0b3.dev0+git.41.g95d28043.tar.gz
WHL file	kolibri-0.17.0b3.dev0+git.41.g95d28043-py2.py3-none-any.whl

[marcellamaki]

does not run any migrations when running the WHL file from the PR, in virtualenv with a new KOLIBRI_HOME