Applications API: add GET and DELETE endpoints

Problem to solve

Applications API currently only offers a create (POST) endpoint. It would be useful to be able to list all applications, and to delete a specific application.

Further details

• Admin user can list all registered applications from the API
• Admin user can delete an application from the API
• Admin user can modify an application (delete+create should be good enough for a first iteration)

Links / references

Past implementation of the POST API: https://gitlab.com/gitlab-org/gitlab-ee/commit/81bbcfacb0adfa32b15b044bfb997aca7bed69fb