Allow finer-grained issue=>channel routing

[infojunkie]

Right now, project issues are routed to Slack channels based on a Mantis project-to-Slack channel mapping. In some instances, finer control over routing may be necessary. For example, Meedan uses different Slack channels for development and product design/management, but the Mantis project is the same for both. We'd like to route dev issues to a channel, and product design issues to another. This may be resolved by routing based on categories as well.

[infojunkie]

Too complex - dropping.