Plugins: Add isEnabled method

Plugins are now disabled at startup time unless there is some relevant
configuration in hydra.conf. This avoids hydra-notify having to do a
lot of redundant work (a lot of plugins did a lot of database queries
*before* deciding they were disabled).

Note: BitBucketStatus users will need to add 'enable_bitbucket_status
= 1' to hydra.conf.

src/lib/Hydra/Plugin/GitlabStatus.pm

@@ -16,6 +16,11 @@ use List::Util qw(max);
 #   - gitlab_project_id => ID of the project in Gitlab, i.e. in the above
 #     case the ID in gitlab of "nixexprs"
 
+sub isEnabled {
+    my ($self) = @_;
+    return defined $self->{config}->{gitlab_authorization};
+}
+
 sub toGitlabState {
     my ($status, $buildStatus) = @_;
     if ($status == 0) {