`disable-message-edit-history`: Remove text about EDITED label
and link to `view-a-messages-edit-history` instead.
`edit-or-delete-a-message`: Reformat 'EDITED' and '(deleted)'
to be bold instead of using backticks. Make link to view
edit history clearer.
Note that the text used in the `OpenGraphTest` in
`test_middleware.py` had to be updated for the changes to
`disable-message-edit-history`.
The message-editing section of settings is moved from "Organization
Settings" to "Organization Permissions", which feels like a more
natural place for these settings.