Fix constraint sync during migration - restore constraints lost during merge in test schema. - use constraint_by_eid in BeforeDeleteCWConstraintHook as done in 3.17.14 for BeforeDeleteConstrainedByHook. Fixes handling of multiple constraints of the same type. - make sync_schema_props_perms() delete the CWConstraint entity instead of the constrained_by relation. In 3.19, the latter doesn't automatically result in the former just because the relation is composite. Simplify the constraint migration to delete all removed constraints and recreate new ones even if they share the same type; that optimization made the code more complicated for (AFAICT) no significant reason.

.. role:: raw-html(raw)
   :format: html

RSS driven
----------

RSS is a pretty useful technology that can be widely used on this
site. Any set of data can be presented as RSS. You can then plug in
an RSS reader into that and follow the site activity. For example :

:raw-html:`<p><a class="reference"
href="view?vid=rss&amp;rql=Any+X%2CM+WHERE+X+modification_date+M+ORDERBY+M+DESC+LIMIT+30"><img
alt="rss" src="data/rss.png"> latest changes</a></p>`