Replace generating preview images with an admin endpoint

It doesn't make much sense to have this per collective, because this
is triggered by an admin after changing the server config file. So it
is now implemented as an admin endpoint that affects all files.

Changelog.md

@@ -1,5 +1,17 @@
 # Changelog
 
+## v0.25.0
+
+*Unreleased*
+
+### Rest API Changes
+
+- Removed `sec/collective/previews` endpoint, in favor for new
+  `admin/attachments/generatePreviews` endpoint which is now an admin
+  task to generate previews for all files. The now removed enpoint did
+  this only for one collective.
+
+
 ## v0.24.0
 
 *Jun 18, 2021*