Add user defined categories/tags to Planned Sessions and Session Templates

[Sander Roosendaal]

Add a field “category” and let users define categories for planned sessions.

Implement filtering on category/tag for session templates.

This helps coaches quickly find types of sessions (Cross-training, Weights, Zone 1, Z2, Z3, etc)

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Perhaps use https://django-taggit.readthedocs.io/en/latest/index.html ?