swh-deposit: Adapt swh.deposit.wsgi starting point

swh-deposit: Use proper configuration scheme swh-deposit: Remove not yet used configuration deposit.env: Add missing required django settings module

Related T1533 Depends on swh-deposit!368 (closed)

Test Plan

doco up -d swh-deposit doco logs --follow swh-deposit everything starts fine

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Migrated from D1160 (view on Phabricator)

conf/deposit.yml

+scheduler:
+  cls: remote
+  args:
+    url: http://swh-scheduler:5008
+
+private:
+  secret_key: prod-in-docker
+  db:
+    host: swh-deposit-db
+    port: 5432
+    name: softwareheritage-deposit
+    user: postgres
+    password: testpassword
+  media_root: /tmp/swh-deposit/uploads