CloudFormation parameter updates

[k-mahoney]

Currently the GitHub repository dispatch tickets are opened in is set via the CloudFormation template. This should be updated such that the repository specified in the template acts as the default or fallback destination for dispatch tickets and allow a repository to be specified in the message specification. This way different dispatch alerts can be ticketed in specific repositories, rather than all defaulting to one.

/cc @alulsh

[alulsh]

We'll also need to add a CloudFormation parameter for the fallback GitHub team, right now this is hard coded to @mapbox/security in https://github.com/mapbox/dispatch/blob/master/incoming/function.js#L113.

[alulsh]

@k-mahoney is going to work on both of these today.

[k-mahoney]

Complete in #104