Add Dangerfile reminder when setting cookies server-side

What does this MR do and why?

If a developer sets a cookie from Rails with a :domain attribute, they must remember to un-set this cookie when a user signs out. This can be hard to remember to do.

This change adds a Danger bot reminder whenever setting cookies is detected in application code, and also documentation of development guidelines when working with cookies.

Primary issue: #477321 (closed)

Previous discussion in: !156213 (comment 2012279812)

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Before	After

How to set up and validate locally

1. Check out a new branch off of this one
2. Add this line of code to any controller action: cookies[:favorite_animal] = 'opossum'
3. Commit the change
4. run rails danger_local to run Danger locally
5. You should see the message It looks like you are setting a server-side cookie. Please note... in the Markdown: section of the output from this command

Related to #477321 (closed)