Implement GitHub logins

Requires the following configuration options
enable_github_login = 1
github_client_id
github_client_secret
Or github_client_secret_file which points to a file with the secret

src/sql/hydra.sql

@@ -10,7 +10,7 @@ create table Users (
     emailAddress  text not null,
     password      text not null, -- sha256 hash
     emailOnError  integer not null default 0,
-    type          text not null default 'hydra', -- either "hydra" or "google"
+    type          text not null default 'hydra', -- either "hydra", "google" or "github"
     publicDashboard boolean not null default false
 );