Squash migrations for v1.4

[mikkonie]

I was originally not intending to do any further migration squashing until e.g. v2.0 or some other very major SODAR Core update.

Alas, #1861 requires this to enable us to get rid of the now-unused django-markupfield dependency. Before doing that, we need to squash migrations and get the user to upgrade their SODAR Core based server to v1.4.

This should be done at the end of v1.4 development before #1862, once we are sure no more new migrations are required.

Also see #1868, can we somehow get around that?