Full restore creates missing tables and suspends foreign keys; --install-apps; clearer messages

Backups now record each table's structure in the dump manifest. The full
restore creates tables that do not exist on the new instance (left behind
by apps that were removed or are switched off), lists all problems at
once, and no longer stops on apps that are missing. Foreign keys are not
checked while rows are loaded in name order, and checked again before the
restore commits (PostgreSQL: dropped and added back in the transaction).

--install-apps installs or enables the apps that were enabled at backup
time; --skip-missing-tables leaves out tables of older backups whose
structure is unknown. nextcloud-aio is never required.

backup:target:add without a key points to backup:key:import, and says that
connecting can take a while. Removing a location drops its cached index;
a migration removes the cache of locations that are gone.

The full-restore test now also runs on PostgreSQL with Mail (foreign keys)
and a disabled app.

.github/workflows/ci.yml

@@ -104,3 +104,5 @@ jobs:
           persist-credentials: false
       - name: Full restore round trip
         run: tests/integration/full-restore-roundtrip.sh 35-apache
+      - name: Full restore round trip on PostgreSQL, with Mail (foreign keys) and a disabled app
+        run: tests/integration/full-restore-roundtrip.sh 35-apache pgsql