[WIP] Elasticsearch collection

[markus1978]

This is an ugly PR:

• lots of changes, due to pulling lots of mongo specifics into entry_collections.mongo
• it supersets the other PR on the elastic transformer
• its still work-in-progress with no tests for the elastic implementation in the CI so far

Please have a look at the mongo stuff and let me know what you think.

[codecov]

Codecov Report

Merging #339 into master will decrease coverage by 4.79%.
The diff coverage is 55.83%.

@@            Coverage Diff             @@
##           master     #339      +/-   ##
==========================================
- Coverage   90.56%   85.76%   -4.80%     
==========================================
  Files          55       56       +1     
  Lines        2268     2438     +170     
==========================================
+ Hits         2054     2091      +37     
- Misses        214      347     +133     

Flag	Coverage Δ
#unittests	85.76% <55.83%> (-4.80%)	⬇️

Impacted Files	Coverage Δ
optimade/server/entry_collections/elasticsearch.py	0.00% <0.00%> (ø)
optimade/server/routers/utils.py	90.24% <ø> (-7.49%)	⬇️
...made/server/entry_collections/entry_collections.py	82.60% <60.00%> (-6.29%)	⬇️
optimade/filtertransformers/elasticsearch.py	89.28% <90.47%> (+1.54%)	⬆️
optimade/server/entry_collections/mongo.py	87.93% <94.73%> (+1.19%)	⬆️
optimade/server/config.py	100.00% <100.00%> (ø)
optimade/server/entry_collections/__init__.py	100.00% <100.00%> (ø)
optimade/server/main.py	84.78% <100.00%> (-0.94%)	⬇️
optimade/server/main_index.py	89.18% <100.00%> (-1.29%)	⬇️
optimade/server/routers/links.py	100.00% <100.00%> (ø)
... and 4 more

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[ml-evs]

Sorry for taking so long to deal with this @markus1978, this PR is now superseded by #613 which incorporates all the changes from this PR and also rebases onto the new base transformer.

[ml-evs]

Just realised that I closed this PR prematurely, apologies! I will be add the changes from this PR into a new PR after we have #613 merged. I will make sure to incorporate and credit your changes here @markus1978.