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