Don't allow content blocks to link to external URLs

A recent change to the URL pattern means these don't resolve.
Adjust order of INSTALLED_APPS to help with custom migration

This helps when customising an app whose initial migration has
dependencies on other apps.  It's not a watertight fix but is backwards
compatible and will generally help.

Fixes #823
Change migration recommendation for custom apps

Related to #823
Merge remote-tracking branch 'upstream/master'

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