Ensure headercolor is always set

[TheLastProject]

Issue #2733 only could have happened because headercolor wasn't set for some cards. While the fix should stand for years, it will undoubtedly frustrate some users to see their card colours change, which will happen if we add new default colours.

We should ensure that headercolor is always set where we can make sure of it. Checking all the importers should be sufficient to at least ensure all newly imported data is correct.

Not sure yet if it's worth to do a full database migration, but may be worth considering to just make unset header colours invalid in the database and assign every unset card a random colour.

[TheLastProject]

Actually, we should consider a database migration. It would be a good permanent fix and would possibly allow us to clean up some code too, simplifying the codebase :)