Update swh-web django applications configuration and setup
Towards reorganizing swh-web features into multiple django applications,
introduce the following changes regarding their settings:
- Declare a fixed set of base django apps and enable to add extra ones through configuration
- Add application templates directory to django templates search paths
- Automatically register URLs of selected django applications
- Inject swh-web django app names in templates to check a feature is enabled
Related to T4398