Refactor to use StoreService instead of Store

[Dassderdie]

• There is now a StoreService that wraps the ngrx-store
• This improves the readability and especially discoverability of synchronous access to the store (storeService.select(selector) instead of getStateSnapshot(selector, store))
• It is now possible to add comments to the store functions (like that we usually don't dispatch actions but propose them via the exerciseService)
• Previously, one always had to import Store<AppState> to get the typings - now StoreService is enough
• Makes it easier to replace ngrx should the need arise as we only use a very small feature set of it

Nearly all of the changes are just straightforward renaming.

[benn02]

LGTM, but this still has conflicts to be resolved. Please resolve them for approval.

[Dassderdie]

I believe I fixed a potential bug here:
If the leaderId would have changed back from an id to undefined, the component would still have displayed the previous leader.