Creating unique constraint & indexes at the db_dbgroup_dbnodes table in SQLA

[szoupanos]

Adding indexes and unique constraints of issue #1678

[codecov-io]

Codecov Report

Merging #1680 into develop will decrease coverage by 0.02%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop    #1680      +/-   ##
===========================================
- Coverage    57.15%   57.13%   -0.03%     
===========================================
  Files          275      275              
  Lines        33917    33919       +2     
===========================================
- Hits         19385    19378       -7     
- Misses       14532    14541       +9

Impacted Files	Coverage Δ
aiida/backends/sqlalchemy/models/group.py	90.62% <100%> (+0.62%)	⬆️
aiida/backends/djsite/globalsettings.py	81.57% <0%> (-5.27%)	⬇️
aiida/backends/djsite/db/models.py	75.09% <0%> (-0.89%)	⬇️

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[giovannipizzi]

Thanks!