backend.

[kohlhase]

Instead of the GIT-based JSON file solution proposed in MathHubInfo/Frontend#88, we probably need a database-based back-end web application that serves JSON, which stores lenses, and a service which stores lens information. We need the following routes:

• components: given a list of lenses and a MMT URI give the list "supported" components at the level. For instance, given a library, give the supported archives and associated confidence values. Given a path, return the directories in an archive.
• add-URI/delete-URI/change-weight-URI: the current URI to a given lens (after authentication)
• list: list all the lists available, maybe we want to have a grouping feature in the future for this.
  I made some lenses in JSON in the data repository for testing. The organization there should influence the database schema, but that need not be identical