Fixed subscription deletion

Added welcome Jumbotron with some nice info/links

Some work on start page

Added feedback_stage_for_user to feedback serializer

This field will contain the feedback stage of the subscription for which the requesting user has created this feedback. This is needed to correctly give the choice to set the final attribute in the frontend.

Fix final checkbox frontend

Fix disappearing names in feedback list

Impplemented skip functionality when correcting

Subscriptions are now deletable

Added tqdm to requierements

Changed jwt auth endpoints

`/api-token-auth/` to `/api/get-token/`
`/api-token-refresh/` to `/api/refresh-token/`

Added student list page

Added submission serializer for student list view

Bare student list view is now implemented for the reviewer
He is able to view a searchable and sortable list of all students in the left pane of the window and their submissions on the right side

Fixed /?#/ bug on login page

Added `feedback_created`/`feedback_validated` counts to TutorSerializer

Added basic tutor overview

Added student overview help card

When going to the student overview page, a help card is shown on the right side which is replaced when viewing a submission

Routing is ow dynmacilly handled by selector components. E.g. the LayoutSelector will render as the correct Layout depending on the logged in users role. This allows to simply route to e.g. '/home/ and the StartPageSelector will determine if StudentPage or TutorStartPage should be displayed.