Migration fixes

[frankystone]

This adds some migrations to fit the latest changes on models like adding/changing the on_delete argument.

• some changes affects only the b-prefix of strings. For those changes i modified the existing migration files
• during testing i found a bug where pybb and threadedcomments used the same global setting DEFAULT_MARKUP. The bug was that pybb uses string based choices whereas threadedcomments uses int-based choices
• another thing was wlscheduling where a default of 0 for a DateTimeField isn't valid (anymore) -> just removed the default

I wanted to make a test by setting up a new website like described in the README, so please wait with merging until my test succeeds.

[frankystone]

All works fine.

On alpha there might be complications applying this branch, because some migrations where added accidentally recently. To fix that one has to remove the accidentally made migrations from the table django_migrations in the mysql console.