Show basic statistics

[nichtich]

The idea came up via #37

Add /stats endpoint with basic statistics

• number of known accounts
• number of known identities per provider
• number of active sessions

[stefandesu]

Alternatively, we could add a stats property for the /about endpoint.

[stefandesu]

Currently, I have implemented it within /about like this:

  "statistics": {
    "userCount": 4,
    "userCountByProvider": {
      "local": 2,
      "github": 1,
      "stackexchange": 0,
      "easydb": 0,
      "wikidata": 1,
      "orcid": 0,
      "testscript": 2
    },
    "sessionCount": 3
  },

Any requests for changes @nichtich ?