Add a relation type "pr_for"

This will add add a relation type `pr_for`.

Use case:
1. A user opens an issue.
2. Another user submits a PR to fix this issue

Now you would mark the PR as `pr_for` the issue.

Real example:
* Issue: joomla/joomla-cms#3370
* PR: joomla/joomla-cms#3700