825 add toggle to add/remove current site to/from element sites

[MyPyDavid]

Description

The new endpoints that add or remove the current site from the sites of an element are:

Django back-end

questions

• PUT /api/v1/questions/catalogs/{id}/toggle-site/

tasks

• PUT /api/v1/tasks/tasks/{id}/toggle-site/

views

• PUT /api/v1/views/views/{id}/toggle-site/

These are defined in a new ViewSetMixin, ElementToggleCurrentSiteViewSetMixin in rdmo/management/viewsets.py. This mixin has one action toggle-site and only for put is allowed.
It adds or removes the current site from the object, depending on whether is was there already or not (like a toggle). For permissions it uses a new (rules-based) CanToggleElementCurrentSite permission class.

The mixin class could be simplified by removing the serializer_class and the get_queryset method. Could we adapt the ElementSucces reducer to not need the serialized object for these actions? Permissions could by checked right in the action methods by eg. test_rule().

React front-end:

• new component: ToggleCurrentSiteLink in rdmo/management/assets/js/components/common/Links.js
• added elementAction to storeElement and storeElementInit
• the url routing is done in eg. the MultiSiteApi.js storeElement via the action

Related issue: #825

Motivation and Context

How has this been tested?

Screenshots (if appropriate)

Logged in as example-editor.

Types of Changes

• [x] New feature (non-breaking change which adds functionality)

Checklist

• [x] I have read the contributor guide.
• [x] My code follows the code style of this project.
• [x] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[MyPyDavid]

need to re-do the commits and add the tests but buttons seem to be working like this

[MyPyDavid]

• [x] the current site toggle should not be affected by the "locked" state of the element

[jochenklar]

Can this be merged?