Add option to filter issues by contributor/participant

Description

Searching for issues would sometimes be faster / easier if I could include the filter / option to only search for issues which I have contributed to.

Proposal

Make it so, whether as filter, or toggle.

Links / references

@andrewn was saying how useful this was, he is my reference on this one :-)

Documentation blurb

(Write the start of the documentation of this feature here, include:

1. Why should someone use it; what's the underlying problem.
2. What is the solution.
3. How does someone use this

During implementation, this can then be copied and used as a starter for the documentation.)

Design

• Add contributor:@user to issue filter dropdown
• Filtering by contributed should show all issues that user has contributed to (comment, emoji, author, assignee).
• Icon is comment-dots