Use 'assign' for PR management

[felixwellen]

I think it would help with managing PRs, if we use the 'assign' feature to select a person, that has to take care of a PR. Where take care means something from the following not necessarily complete list:

• Review it
• Find an appropriate reviewer for the PR, request a review and assign it to the reviewer
• Ask a blocking question, and if there is a unique person to answer the question, assign to them.
• Close the PR, if it the right thing to do
  The advantages are the following:
• There are more things that can be done with a PR than reviewing, the 'assign' feature seems to be the way to implement that.
• There is good support from github, for showing which PRs are assigned/unassigned (not the case for reviewers).