signal handlers for syncing of project views and tasks

[MyPyDavid]

Description

For the settings this PR removes:
PROJECT_REMOVE_VIEWS = True
and adds:

PROJECT_VIEWS_SYNC = True
PROJECT_TASKS_SYNC = True

The projects/handlers.py are refactored into a project/handlers package with a project/handlers/generic_handlers.py module that contains the logic for signal receivers.
It handles the signals for changes in View.catalogs, View.sites, View.groups as well as Task.catalogs, Task.sites, Task.groups and updates the project.views or projects.tasks.

Tests, that only use db view.catalog.add(...) methods and not the client, are included.

Related issues: #966, #1198, #345, #431
Related PRs: #1200

Motivation and Context

ToBeDone:

• Add tests
• Decide on automatically adding views to projects (maybe also enabled by setting?)
• Change to custom signal/include filtering of views based on user permissions
• Implement adding of views to other signals.
• ...

How has this been tested?

Screenshots (if appropriate)

[jochenklar]

I like this. We should stick to helpers for tests (and use sparingly) and utils for regular code.

[jochenklar]

I don't think those methods should exist, this should be inline in the handlers (and without the getattr).

[MyPyDavid]

I could make it more explicit but it would require a lot more boilerplate code.
The signal handlers for View and Task are basically duplicates, so there would be 18 branches instead of the 9 if action == ... in total in the different functions for .catalogs, .sites and .groups .
But we already agreed about this in the meeting right?

Or would there be an extra use case that would require an update on the Issues as well, when project.tasks are handled?

Example for an explicit function

    # Identify the catalogs affected by this change
    catalogs = model.objects.filter(pk__in=pk_set)

    # Determine the projects to be modified
    if action == 'post_add':
        projects = Project.objects.filter_catalogs(catalogs).exclude(views=instance)
        for project in projects:
            project.views.add(instance)

    elif action == 'post_remove':
        projects = Project.objects.filter_catalogs(catalogs).filter(views=instance)
        for project in projects:
            project.views.remove(instance)

    elif action == 'post_clear':
        projects = Project.objects.filter(views=instance)
        for project in projects:
            project.views.remove(instance)

[MyPyDavid]

I think that a Project should be able to find the views that are available for itself. Do you agree and would this query be correct for that?
I've changed the filter_group a little bit to handle a list of users as well as a single user.

[MyPyDavid]

I tried to prevent calling the save method if nothing was changed

[MyPyDavid]

this was just still missing right? ;)

[MyPyDavid]

I needed a pre_save to check if the catalog was changed or not. And afterwards it passes this boolean (with deferred name) to the post_save.
It seems complicated for just a project.save() but could not find another way.

[MyPyDavid]

I wanted to explicitly test this filter filter_available_views_for_project with this mapping but have not implemented that test yet.