Manage tasks fetching with react-query and remove tasks from the store

I tried to use react-query for a simple usecase (fetching tasks) to get a feel of it and see if it was worth moving from rest-hooks to react-query. I must say that it was quite straight-forward and I appreciated that.

I have to see how it will fare for more complex usecase.

It also allowed me to remove the tasks store slice (#185 (closed)) since tasks are now stored in react-query fetch store.