The `no-store` cache control added in #20432 is causing form input to be
cleared unnecessarily on page reload. Instead use
`max-age=0,private,must-revalidate` which avoids this.
This was particularly a problem when typing a long comment for an issue
and then for example changing the label. The page would be reloaded and
lose the unsubmitted comment.
Fixes#22603
1e0e79dcbf