Set up app setting definitions as objects

[mikkonie]

As with many return values of implementable plugin methods, expecting these as dictionaries offers a lot of chance for user error and ambiguity.

We should set up a class for app settings and expect definitions to take form of a list of objects instead of a dict.

This also allows us to place critical validation in the class __init__() to detect badly defined settings as early as possible.

Providing app settings as a dict should be deprecated with warning in v1.1 and removed in v1.2.

Existing setting dicts within SODAR Core should be replaced with the class objects, of course.

Tasks

• Add PluginAppSettingDef class
• Update usages and existing definitions
• Allow dict definitions with deprecation warning
• Fix test failures from initial commit
• Fix [Not editable on target sites] label visibility
• Rename misleading s_val variables
• Remove redundant s_key variables
• Add default value validation on model level
• Add Django check for name uniqueness within plugin
• Update tests
• Add model tests for PluginAppSettingDef
• Update docs and examples

[mikkonie]

Done.