[ADD] various: deduplicate soon-to-be-unique fields before migration

Some target versions add a ``UNIQUE`` constraint on a field that allowed
duplicates in the source version (e.g. ``utm.source.name`` in 16.0). The
unique index creation then aborts the whole registry load with a
``psycopg2.errors.UniqueViolation``.

Declare such constraints per version in ``known_changes.yaml`` under the
``new_unique_constraints`` key (only ``model`` and ``fields`` needed).
During ``prepare_db.sh``, ``dedup_unique_constraints.py`` aggregates the
declarations of every traversed version and, on the source database,
renames duplicates by suffixing `` [<id>]`` while preserving foreign keys.

The actual PostgreSQL column type is used (not the ORM ``translate``
attribute) so both ``varchar`` and already-converted ``jsonb`` columns are
handled: ``jsonb`` duplicates are compared and renamed on every language
key (indexed ``en_US`` value). Renamed records are reported in a
timestamped JSON file and summarized at the end of ``migration.log``.

versions/16.0/known_changes.yaml

@@ -21,3 +21,12 @@ renamed:
     new: server_action_mass_edit
   - old: crm_project
     new: crm_lead_to_task
+
+# Unique constraints newly introduced in 16.0
+new_unique_constraints:
+  - model: utm.source
+    fields: [name]
+  - model: utm.medium
+    fields: [name]
+  - model: utm.campaign
+    fields: [name]