Allow Plugins to register a list of Django apps to be appended to INSTALLED_APPS

[jsenecal]

Fixes: #9880

This PR extends the PluginConfig class to support a new django_apps attribute which would get appended to INSTALLED_APPS.

Like so:

from extras.plugins import PluginConfig

class FooBarConfig(PluginConfig):
    name = 'foo_bar'
    verbose_name = 'Foo Bar'
    description = 'An example NetBox plugin'
    version = '0.1'
    author = 'Jeremy Stretch'
    author_email = '[email protected]'
    base_url = 'foo-bar'
    required_settings = []
    default_settings = {
        'baz': True
    }
    django_apps = ["foo", "bar", "baz"] ### <<< HERE

config = FooBarConfig

These new apps should be inserted before the plugin that requires it, but after existing "core" apps in order to avoid conflicts. If the plugin's PluginConfig is in the supplied list of modules, then the list is appended as-is to the INSTALLED_APPS. If, however, the plugin name is added in there, it will be silently removed and its PluginConfig will be added instead.