Make links/mappings to repository changeset configurable

When integrating JIRA with subversion or git repositories, currently it is only possible to link to JIRA issues by mentioning the JIRA issue key in commit messages/change sets. There are advantages in making this more configurable, such that one could specify some other property e.g. a custom field that could be used for the link other than issue key if necessary.

The use case for this is usually with users migrating from other issue trackers with pre-existing subversion integration. We may be seeing more requests for this e.g.:

• https://answers.atlassian.com/questions/282164/how-to-index-repository-referecing-ids-from-migrated-bugzilla-issues
• https://answers.atlassian.com/questions/174771/how-to-receive-subversion-commits-of-an-imported-project-using-the-external-issue-id

This applies to different methods of integration including JIRA Subversion plugin, Fisheye plugin, DVCS connector etc.