Cleanup conversation headers #3089

PR headers have a lot of duplicate information:

<img width="652" alt="" src="https://user-images.githubusercontent.com/1402241/81815942-ee748480-952a-11ea-8155-7968cd9ca183.png">

Drop:

• Base branch, if it's the default
• User, if it matches the Author
• Commit numbers: it appears in the tabs just below
• The verb: it implied and sometimes duplicate

Merged (before and after)

<img width="738" alt="" src="https://user-images.githubusercontent.com/1402241/81815337-23340c00-952a-11ea-9609-c0a73daad7ca.png">

<img width="623" alt="" src="https://user-images.githubusercontent.com/1402241/81815495-55456e00-952a-11ea-9f71-3d3850c59802.png">

Open

<img width="752" alt="" src="https://user-images.githubusercontent.com/1402241/81816232-57f49300-952b-11ea-9b59-3ff03a2034cd.png"> <img width="742" alt="" src="https://user-images.githubusercontent.com/1402241/81816237-5925c000-952b-11ea-9edc-2fd23d3b5660.png">