Add ability to clear cachePath and capturePath

[joe-allen-89]

Subject of the issue/enhancement/features

Currently the cachePath/captureDir is not being cleared during the migration process, when using migrations on the authoring tool cache files are building up quite quickly and capturePath is being cleared within the tools functionality.

Solution is to add migration:clear which would remove both the cachePath and captureDir within the grunt task which can be easily called from both framework and authoring tool.

[oliverfoster]

Did you have a look at the CacheManager at all?

https://github.com/adaptlearning/adapt_framework/blob/df5e2251242b791cbf748b700b4c39774a51c974/grunt/tasks/migration.js#L41-L45

https://github.com/adaptlearning/adapt-migrations/blob/4060bc96fff7dd5817e9320397d67e9991292cb0/lib/CacheManager.js#L15