Add more information about an issue

[yoyozbi]

Is your feature request related to a problem? Please describe.
The current implementation is missing some information that could be found in a gitlab issue (labels, milestone, weight, time tracking and health status, assignees, issue project number (the #3 thing, I don't know how to call it)).

Describe the solution you'd like
Implement these new fields to be available through handlebars templates (only text could be required)

Describe alternatives you've considered
Don't implement some of them, if maybe they are not available for GitLab CE or check for the GitLab version before populating fields.

Additional context
I could also try to do it

[benr77]

Sure, PR welcome - probably simplest to only include fields that are available on all Gitlab versions. Thanks.