Draft: Read previous datetime from env

Related to management#821 (closed)

Reimplement incremental mode reading "FROM_DATETIME" env variable instead of opening the previous table of contents XML file in Git.

TODO before merging

• restore .gitlab-ci.yml

.gitlab-ci.yml

@@ -21,7 +21,7 @@ job:
 
     # Enter virtualenv and install fetcher dependencies.
     - source ${FETCHER_ENV_DIR}/venv/bin/activate
-    - "[ -f requirements.txt ] && pip3 install --requirement requirements.txt"
+    - '[ -f requirements.txt ] && pip3 install --requirement requirements.txt'
 
     # Run ssh-agent (inside the build environment).
     - eval $(ssh-agent -s)