Add default Derivation Group associations for Plans at the Model level

[pranav-super]

This is the UI update based on the tables introduced in this Plandev PR.

It introduces a derivation group tab along with the other model association tabs on the models page.
This feature is modeled off of the other associations, but since derivation group associations are much simpler than goal/condition/constraint specifications, is a lot lighter-weight and doesn't precisely match functionality.

There are a few minor unresolved TODOs, namely:

• deleting an external source gets blocked if there are any outstanding plan-derivation-group links.
  We don't believe this is a necessary check for model-derivation-group links (i.e. if a derivation group is linked to a model, and a planner wants to delete a source from said group, they shouldn't be blocked from doing so). We believe this is an unnecessary check because with a model, there are no activities scheduled off of any sources or external events visible to planners at a model level, so there is no anticipated loss when a source is deleted.
• a few formatting issues with the derivation group table on the models page. The table is modeled off of the ManagePlanDerivationGroupsModal, and a lot of the formatting and divs were retained as attempts to format otherwise were unsuccessful.