Introduces a basic CSV import module for bulk-importing account transactions.
Changes include:
- User can load a CSV
- User can configure the column mappings for a CSV
- Imported CSV shows invalid cells
- User can clean up their data directly in the UI
- User can see a preview of the import rows and confirm import
- Layout refactor + Import nav stepper
- System test stability improvements
* Add climate_control gem and test helper
* Replace ENV mods in upgrades test
* Replace ENV mods in registrations test
* Remove ENV references in hostings controller
* Update ENV refs in mailer test
* ActiveStorage cleanup
* Consolidate queue config so appropriate adapter runs in test environment
* Make test environment more explicit
* Centralize self hosting config
* Remove flaky system test
* Fix merchants controller actions
