Convert logging plug-in system to an extension point

[Nicklas Nordborg]

The ​documentation explains the current system. It is probably not that hard to convert this to an extension point. The added benefit is that it will be possible to use multiple logging implementations at the same time.

[Nicklas Nordborg]

(In [6079]) References #1601: Convert logging plug-in system to an extension point

This should now be implemented and the database log manager has been converted. Need to document changes and also add a note about backwards (in)compatibility.

[Nicklas Nordborg]

(In [6080]) References #1601: Convert logging plug-in system to an extension point

Updated the documentation. Implemented a few utility methods that can be useful when implementing other logging implementations.

[Nicklas Nordborg]

(In [6081]) References #1601: Convert logging plug-in system to an extension point

Added a note in the upgrade instructions about this change and how it affects custom logging implementations.

[Nicklas Nordborg]

(In [6086]) References #1601: Convert logging plug-in system to an extension point

The test code didn't work since it still used the old system. It has now been converted to the extensions system.