One of the main feature that the SWH web user interface will have to offer is the easy browsing of a source code tree (like any software forge web interface).
As a first task for the beginning of the development of the swh web ui, let's implement it.

We will reuse the service module from the swh web api (will need to move it in the common sub-module at the root of swh-web) under the hood as its current version enables to:

• list a directory content
• get the content of a file listed in a directory: