Improve handling of MongoDB ObjectID

[ml-evs]

This PR moves the BSON ObjectID handling code out of the general mapper and into the mongo-specific entry collection. It also adds a post-processing method to the MongoTransformer to handle queries on the underlying Mongo _id (e.g. if you aliased immutable_id to _id) by converting them to the corresponding {"$oid": {"$eq": "123456"} query, disallowing other string filters. This means it can now be aliased in the same way as any other field.

• The test server now uses _id as the underlying field for the immutable_id OPTIMADE field.
• MongoDB projection is now done in a dict format to allow for explicit exclusion of _id if it has not been aliased (by default, our existing list-based projection always includes _id, which is why we had to delete it every time in the mapper).

[codecov]

Codecov Report

Merging #557 into master will increase coverage by 0.03%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master     #557      +/-   ##
==========================================
+ Coverage   91.48%   91.51%   +0.03%     
==========================================
  Files          62       62              
  Lines        3112     3135      +23     
==========================================
+ Hits         2847     2869      +22     
- Misses        265      266       +1     

Flag	Coverage Δ
#project	91.51% <96.55%> (+0.03%)	⬆️
#validator	63.41% <58.62%> (-0.12%)	⬇️

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Impacted Files	Coverage Δ
optimade/server/mappers/entries.py	98.30% <ø> (-0.06%)	⬇️
optimade/server/entry_collections/mongo.py	95.31% <50.00%> (-1.47%)	⬇️
optimade/filtertransformers/mongo.py	97.36% <100.00%> (+0.21%)	⬆️
...made/server/entry_collections/entry_collections.py	98.85% <100.00%> (+0.02%)	⬆️
optimade/server/exceptions.py	100.00% <100.00%> (ø)

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[ml-evs]

Made a couple of tweaks here too, in 068c3b3, which adds a mixin for HTTPExceptions that defines __str__() to be the same as __repr__ for easier use with pytest.raises. I don't think there will be any other side effects from this but I'll leave it until tomorrow to merge this one...

[ml-evs]

Hmm, either this PR or one of the more recent ones should have triggered a test failure during validation, as our test server responds with BadRequest (rather than some nasty pymongo error) when requesting e.g. immutable_id STARTS WITH 'blah'. I'm going to have to make another PR that changes the BadRequest to NotImplementedError so that it can pass our validator.