Add url replacement helpers for Private Attachments

In https://github.com/humanmade/S3-Uploads/pull/398 we merged the first part of private attachments implementation.

When you use private attachments, it's quite likely that private / temporary URLs can get stored in the post content. We should add some helper functions for adding / removing and refreshing temporarily urls in post content / arbitrary strings.

Start by reading pull request 398 and the existing private attachments implementation; the issue does not name specific files or tests. Trace how temporary URLs enter post content and arbitrary strings, then define coverage for adding, removing, and refreshing them before checking how completion can be verified.

Written by the indexing model from the issue text.