Move all shared code to frontend/src/shared & divide types into separate folders

[ShootingStar91]

No description provided.

[ShootingStar91]

• Moved backendTypes.ts from frontend/src to frontend/src/shared/types as separate files. All shared types are now imported from that folder, but they are divided into separate files that are imported via index.ts.
• frontend/src/types.ts was removed, it had some calculation-functions which are now in shared/calculations.ts, as well as one type which is in types folder
• fixed the copied types in reference validator to be correctly imported instead of copied

Merge this if you want, or if it disrupts your workflows too much you can leave it and I will merge after your group is done. But it'd be nice if you took a look at it in case you have opinions on any changes.

[iritmaximus]

LGTM. That was a lot of files changed, took a while to even scroll all of them

[iritmaximus]

This seemed like a clear improvement.