Code changes required for integration with a local PlantUML server

MR gitlab-ce!30199 left two issues to be resolved. As they don't relate to documentation, I have raised this issue to have them addressed:

• plantuml should be added to the blacklist of group names

  Thread: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30199#note_194576803

• plantuml should be added to TOP_LEVEL_ROUTES in this file:

  https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/gitlab/path_regex.rb

  Thread: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30199#note_194620484

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Proposed Fix (from this note):

1. Make the path /-/plantuml/ so it's explicitly not a group or user path.
2. Add plantuml to the - scope in https://gitlab.com/gitlab-org/gitlab-ce/blob/master/config%2Froutes.rb so that we don't try to use it in the application in future. (We can just make it an explicit 404 in the routing.)