Move RunnerConnection into class Runner

The old approach was to require the runner connection. This did
not work anymore with Zeitwerk in Rails 6. @sebastian.serth and I
moved the Connection class in `lib` into the ActiveRecord class
`Runner`. This will also work with future changes like specific
error classes. Furthermore the config was fixed and simplified.

Co-authored-by: Sebastian Serth <Sebastian.Serth@hpi.de>

config/code_ocean.yml.ci

@@ -11,5 +11,4 @@ test:
     enabled: false
   runner_management:
     url: https://runners.example.org
-    cleanup_interval: 60
     unused_runner_expiration_time: 180

config/code_ocean.yml.example

@@ -4,27 +4,25 @@ default: &default
     answers_per_query: 3
   code_pilot:
     enabled: false
+    url: //localhost:3000
   codeharbor:
     enabled: false
   codeocean_events:
     enabled: false
+  prometheus_exporter:
+    enabled: false
   runner_management:
     url: https://runners.example.org
-    cleanup_interval: 60
     unused_runner_expiration_time: 180
 
 development:
+  <<: *default
   flowr:
     enabled: true
     answers_per_query: 3
-  code_pilot:
-    enabled: false
-    url: //localhost:3000
   codeharbor:
     enabled: true
     url: https://codeharbor.openhpi.de
-  prometheus_exporter:
-    enabled: false
 
 production:
   <<: *default
@@ -33,5 +31,3 @@ production:
 
 test:
   <<: *default
-  prometheus_exporter:
-    enabled: false