PR: don't display Reopen button for PRs where src or destination branch was deleted

jens@atlassian.com I think we need PM input here - if the source branch has been deleted, they may just want us to re-cretae the branch if it's not there anymore.

If we don't, we need to have a UI for why we're not allowing them to re-open the PR - yet another tipsy? We will need this anyway if the destination branch os gone...

I prefer just recreating the source branch if it's gone.