Add sortable field to EntryInfoProperty model

[CasperWA]

Fixes #84

Also adds a validator for EntryInfoResource, checking that sortable is either there for all or for none of the properties.

[codecov]

Codecov Report

Merging #85 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
+ Coverage   84.31%   84.32%   +<.01%     
==========================================
  Files          35       35              
  Lines        2098     2099       +1     
==========================================
+ Hits         1769     1770       +1     
  Misses        329      329

Impacted Files	Coverage Δ
optimade/models/entries.py	100% <100%> (ø)	⬆️

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[CasperWA]

The sortable validator has been removed. See discussion and reasoning here.

[ml-evs]

👍