Allow mapping by id

[infojunkie]

Right now, mapping projects to channels is done via project and channel names. However, these names can be modified without losing the identity of the underlying entities, causing the mapping to become invalid.

Suggest to allow mapping by either name or id, to make it a little more robust.

[AmauryBs]

Hello, I've added a way to map channels with bug handlers and with bug severity on my fork. I think it would be great to implement this to the plugin

[infojunkie]

Thanks! Can you point to your code for review? Also, I'm out on vacation for 3 weeks so I'll be slow to respond.

[AmauryBs]

Here is my code: https://github.com/AmauryBs/MantisBT-Slack (I also made a pull request). I've changed mainly changed the get channel function. I also added a check box to activate or not those fonctionnalities and a print of the actual configuration in the configuration page. I wish you nice vacations =)