SimpleFin (#2188)

* Some initial UI work for adding SimpleFin.

* SimpleFin proof of concept working.

* Adds linking & unlinking to existing accounts through the account menu UI.

* Added loading and lint fixes.

* Lint changes.

* Added release notes.

* Typecheck cleanup.

* Import, lint, typecheck cleanups.

* More typecheck cleanup.

* Refactored language for consistency.

* Added default institution name.

* Lint cleanup.

* Addressed change requests.

* Added a default to migration, made variables consistent, added feature flag.

* Added account_sync_source to server schema.

* Adds account_sync_source to test.

* Fix for typecheck.

* Attempt to make typecheck happy.

* Added strict ignore.

* Moved account_sync_source to the right model (face palm).

* Hotfix for institution format.

* Lint cleanup.

* Removed unnecessary promise.all.

* Lint cleanup.