INIT integration: define `InitGroup` model

[angela-tran]

We need to store the values for the MOBILEvario FareCategorys that go with each transit benefit type (e.g. Older Adult, Medicare, Veteran) so that we can use them when attempting the card registration for a given transit benefit. The simplest path is to define an InitGroup model, similar to the LittlepayGroup and SwitchioGroup models.

There are some GitHub issues related to improving the modeling of LittlepayGroup (#3649) and SwitchioGroup (#3832). It may be tempting to use this task as an opportunity for incorporating some of those ideas since, as the thought might go, we'll already be touching code adjacent to them. I don't think we should spend time on that while we're working on the initial iteration of getting INIT integration implemented, but if we have time at the end, it could be good to take a look at those improvements. I'm open to differing opinions or suggestions on this.

Acceptance Criteria

• InitGroup is defined with respect to what's described above
• InitGroup models can be created / deleted / viewed / changed in Benefits Administrator with the expected permissions respected for our user types of superuser, Cal-ITP staff user, and transit agency staff user
• Sample fixtures updated
• Shared LastPass fixtures updated

[jgravois]

I don't think we should spend time on that while we're working on the initial iteration of getting INIT integration implemented

i agree with this

-[ ] Sample fixtures updated

• Shared LastPass fixtures updated

i like the idea of using a new dedicated encrypted note for collaboration while we experiment, but my instinct is that we'd be best served to keep "[COMBINED] Benefits fixtures" in LP ignorant of our new INIT models until the new proposed model shape is more finalized.