Support use of variables in pull request description templates

Currently, the only special variable that can be used when creating a pull request template is the commit_messages as described in the Pull request and Merge settings documentation.

This feature request is to also support the usage of additional variables to reference other attributes of the pull request : 

• pull request number
• source branch
• target branch
• a short list of included commit messages (first line only)
• an extended list of included commit messages (full commit messages)
• optionally include author/committer, date of each included commit