Issue relationships

[Renegade]

Since we recently had an episode of several requests succeeding each other as the "current" version of the actual request, I've looked into relationships and their customization options, and will make a few changes to the tracker.

For one, I finally got the relationship graphs running, allowing you an actual "overview" of how exactly a given issue is related to other issues on the tracker.
For #221 "Trenches", that looks like this:
   

Secondly, I will add a few new relationships - currently planned are

• "is tracking bug of"/"has tracking bug"
  
• "is update of"/"has update"
  
• "is blocker of"/"is blocked by"
  
• "is requirement of"/"requires"
  

feel free to suggest more.

Unfortunately, it doesn't seem like I can give normal users the right to set relationships, but even for passive consumption, the new relationships should make the web of issues clearer to understand.

DCoder edit: you accidentally a whole verb.