expose trigger configuration schema

Note that this commit contains the following breaking changes:

* remote pull and remote push triggers are now enabled by default (with
  empty target list)
* remote pull and remote push triggers now require target option to be
  set (old behaviour had fallback on `gitremote`)
* validation is now considered to be stable, so it is enabled by default
  in docker image (can be disabled however)

tests/testresources/core/ahriman.ini

@@ -45,7 +45,7 @@ push_url = https://github.com/arcan1s/repository.git
 pull_url = https://github.com/arcan1s/repository.git
 
 [report]
-target =
+target = console
 
 [email]
 host = 127.0.0.1