Locations: edit connection settings without losing the history

A changed password or host name could only be handled by adding a new
location, which starts an empty repository and, with a retention lock at
the provider, keeps the old one billed. Edit now changes a location in
place (web interface and occ backup:target:edit). The settings are only
saved when they lead to the same repository; a failed connection and a
missing repository give separate errors, and the folder is not created
while checking. Stored passwords are never sent to the browser: an empty
password field keeps the stored value.

CHANGELOG.md

@@ -9,6 +9,14 @@ instead carried by the app description, the git tag, the release, and this file.
 
 ## [Unreleased]
 
+### Added
+- Edit a backup location (web interface and `occ backup:target:edit`): change the password, host,
+  folder, connection type or name without losing its history. Before this the only option was a
+  new location, which starts an empty repository and, with a retention lock at the provider,
+  keeps paying for the old one. The new settings are only saved when they lead to the same
+  repository; stored passwords are never sent to the browser, and an empty password field keeps
+  the stored one. Administrators are notified of the change.
+
 ### Changed
 - App store listing: the homepage link points to the GitHub repository instead of back to the
   app store page itself.