fetching of GH ticket needs to be set per-project

[wolftune]

There's currently a "failed to fetch github tickets" error at https://snowdrift.coop/p/taskcoach/t

That happens because, like the snowdrift project, it is possible for any project to fetch tickets from GH. But we need to change this so that it doesn't try automatically to fetch, only if a project has specified a GH address to fetch from.

So:

• turn off fetching if a GH address to fetch isn't set
• create a mechanism in project profile to specify a GH address to fetch from (this may require first creating the whole concept of editable profiles for projects)