Automatically delete head branches of pull requests

You can now select multiple lines in a diff view and link to the lines selected. Previously, multi-line selection only worked when viewing a file.

The retention policy for web notifications will be updated in multiple phases with the goal of retaining all `read` and `unread` messages for a total of three months each.

The first phase of this update has landed which has increased the retention for `read` notifications from 1 week to 1 month and reduced the retention for `unread` from 6 months to 5 months.