v0.4.0

Due to rebase and commit for v0.3.0, some commits will be redone. This
cannot be undone without losing pointers to the v0.3.0 published commit.
One should either merge with --ff-only from command line or make a merge
commit and then merge back into develop using a merge commit as well.
This update is considered from commit 12d2237.

Updates:

• OPTiMaDe spec v0.10.1.
• optimade-python-tools v0.3.1 (with grammar for v0.10.1).
• Redone HTTP middleware to handle session per request.
• Update to pydantic v1.
• Add implementation information, i.e., information about which version
  of aiida-optimade the server is using, to all meta responses.
• Use server.cfg.
• Fix broken Docker build, and add it to CI (@ltalirz)