Allow for the hunt to branch into multi-streams of levels and support independent hints for users

[AdmiralGT]

Here are all my changes to allow levels to have both multiple solutions but also to lead to multiple levels which can then either continue separately or merge back into a single level.

I've done quite a lot of refactoring to add unit tests, these can be run using pytest, I've updated the instructions in the README for this.

I've also tested locally, although the DropBox module doesn't support uploading files from a Windows filesystem so I haven't been able to check that levels display hints correctly.

Things of note:

• I've removed Private hints since independent hints can be modified by an admin to provide the same functionality (simply increase the User Level hints shown by 1).
• I'm not confident on the rubber banding releasing of hints but I think this needs wider refactoring to support Rob's penalty system so not worth changing now.