New Visualization: Issue and PR activity survival analysis

[cdolfi]

[mariashev]

I've been able to create this visualization (PR #741 ) with the exception of the "issue first comment" plot.

Issue: Line stuck at zero because first comment is the initial issue author comment and the contributor id's are not consistent throughout issue reporter_id, issue cntrb_id, and msg cntrb_id - can’t apply the same logic to identify the first comment as has been previously applied for pr's (eg. pr_first_response visualization)

Possible fix: Disregard first comment of each issue - seems like bad practice and doesn’t address possibility of more author comments