Preseed dist with content item db

[benjaoming]

Fix for #4802

[benjaoming]

@rtibbles would you be able to comment on the most recent test failure?

[rtibbles]

Seems to be flakiness in the Item Rating Behave tests - @MCGallaspy any ideas?

[rtibbles]

Otherwise, nothing hugely problematic here, except that content data should be coming from an external source, not our old json files.

If we want to just have a content database to seed the dist with, we should probably just have a dummy fixture - as none of the content will be available, except the Khan Exercises (not Perseus).

[MCGallaspy]

Seems to be flakiness in the Item Rating Behave tests - @MCGallaspy any ideas?

I guess it times out waiting for some condition to be fulfilled. Same step passes in another test. I say merge anyway, and we'll address the test separately.

[benjaoming]

There were issues about this approach, so we need to implement content DBs from the content packs before this can be used. Manually merging stuff from this PR into #4865.