Add ability to have multiple ticket regular expressions

If a company uses multiple ticket types (e.g. when transitioning to a new system), then commit messages might need to have two or more specifications for ticket numbers. We should support this somehow.

Start by locating the existing ticket regular-expression configuration and the code that examines commit messages for ticket numbers. Determine how multiple ticket types should be represented and combined, then verify that commits matching each supported expression are recognized without breaking the current behavior.

Written by the indexing model from the issue text.