Webadmin CRUD for mail repositories

We need an API for viewing/clearing mail repositories. Reprocessing will be handled separately.

• Here are the proposed endpoints:

GET /mailRepositories
GET /mailRepositories/:name list email keys. Also, note the name is a URL itself thus need to be encoded
GET /mailRepositories/:name/:key cf WebAdmin mail format
DEL /mailRepositories/:name/:key
DEL /mailRepositories/:name

Being an URL by itself, the name parameter will need to be encoded.

• We need to write tests for these Routes. We can rely on the InMemoryMailRepository
• Bind it in Guice and write a webadmin integration test demonstrating it works.