Save config in cache when starting the app and when modifying config

[gabegma]

Resolve #345

Description:

• I took the opportunity to do some refactoring so we can skip the validation tasks and start-up tasks in some cases. For now, this is only useful in the update_config route, in the except condition.
• This refactoring triggered some new mypy warnings that I have fixed.

Checklist:

You should check all boxes before the PR is ready. If a box does not apply, check it to acknowledge it.

• ISSUE NUMBER. You linked the issue number (Ex: Resolve #XXX).
• PRE-COMMIT. You ran pre-commit on all commits, or else, you
  ran pre-commit run --all-files at the end.
• USER CHANGES. The changes are added to CHANGELOG.md and the documentation, if they impact
  our users.
• DEV CHANGES.
  • Update the documentation if this PR changes how to develop/launch on the app.
  • Update the README files and our wiki for any big design decisions, if relevant.
  • Add unit tests, docstrings, typing and comments for complex sections.

[JosephMarinier]

Very cool! That paves the way for a lot of cool features. 👍