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)

Dockerfile

@@ -12,7 +12,7 @@ ENV AHRIMAN_REPOSITORY="aur-clone"
 ENV AHRIMAN_REPOSITORY_ROOT="/var/lib/ahriman/ahriman"
 ENV AHRIMAN_UNIX_SOCKET=""
 ENV AHRIMAN_USER="ahriman"
-ENV AHRIMAN_VALIDATE_CONFIGURATION=""
+ENV AHRIMAN_VALIDATE_CONFIGURATION="yes"
 
 # install environment
 ## update pacman.conf with multilib