HFI Station Admin: Edit Station Default Fuel Type

As a HFI Admin I want to edit the default fuel type of a station within a planning area So That all future prep periods will start off with this default fuel type, just for the specified planning area.

**Additional Context**
- `PlanningWeatherStation` model
- [FIGMA](https://www.figma.com/file/FReMPNBDzxYeE8hlxRvsoB/HFI-Calc-(Prepski)-2022?node-id=1473%3A5133)
- User permission required for this: station_admin (probably?)

**Acceptance Criteria**
- [ ] Given I select a station in a planning area, When I change the default fuel type (in the station admin modal), Then it's persisted as the default for that planning area and station
- [ ] Given I've changed the default fuel type for a station, When I select a prep period in the future, the newly selected fuel type is set for that (planning area, station) combo as the default. (Use this as a test case)

**Definition of Done**
- Ready to Demo in Sprint Review
- Does what I have made have appropriate test coverage?
- Documentation and/or scientific documentation exists and can be found
- Peer Reviewed by 2 people on the team
- Manual testing of all PRs in Dev and Prod
- Merged

Start with the PlanningWeatherStation model and the station admin modal, using the linked Figma design to understand the intended interaction. Trace how a station and planning area are selected, then use the future prep-period acceptance case to verify that the persisted fuel type becomes the default for that combination.

Written by the indexing model from the issue text.