fix in custom schema (rest api)

[waychal]

Fixes #1390

[ltalirz]

@waychal First of all, thanks for the fix!

Is the idea that the definition of the projections will move from the custom schema inside the _default_projections of the translator class?
I guess this seems like a reasonable place to define them to make sure that people don't forget them when they add a new translator.

[codecov-io]

Codecov Report

Merging #1490 into release_v0.12.1 will increase coverage by <.01%.
The diff coverage is 65.62%.

@@                 Coverage Diff                 @@
##           release_v0.12.1    #1490      +/-   ##
===================================================
+ Coverage            54.42%   54.42%   +<.01%     
===================================================
  Files                  246      246              
  Lines                32454    32478      +24     
===================================================
+ Hits                 17663    17677      +14     
- Misses               14791    14801      +10

Impacted Files	Coverage Δ
aiida/restapi/translator/node.py	46.89% <37.5%> (-0.33%)	⬇️
aiida/restapi/translator/user.py	80.95% <75%> (-5.72%)	⬇️
aiida/restapi/translator/group.py	80.95% <75%> (-5.72%)	⬇️
aiida/restapi/translator/computer.py	85.71% <75%> (-7.62%)	⬇️

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[waychal]

@ltalirz part of the custom_schema will move to the ORM classes like help_text and display_name as they are more related to the node. is_display and ordering is required only in rest api.. so that part might go to the translator. I still need to finalise that.

[waychal]

@ltalirz there is separate issue for moving custom_schema to orm classes and it requires time to solve that. Until then I guess its good to marge this pr to make the functionality working in current code.

[ltalirz]

Sounds good!