Remove hard coded 'Subset' string checks

[rosteen]

Does what the title says.

[codecov]

Codecov Report

Attention: 10 lines in your changes are missing coverage. Please review.

Comparison is base (0673de8) 91.54% compared to head (7feaf19) 91.54%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
jdaviz/core/template_mixin.py	93.38%	8 Missing ⚠️
...configs/cubeviz/plugins/moment_maps/moment_maps.py	90.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2611   +/-   ##
=======================================
  Coverage   91.54%   91.54%           
=======================================
  Files         161      161           
  Lines       19757    19837   +80     
=======================================
+ Hits        18086    18160   +74     
- Misses       1671     1677    +6     

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[pllim]

Basically all the checks are now double loops (one loop to find all the subset labels and another loop to check if a given label is in those labels). Is there a smarter way to check whether something is a subset without using label at all? 💭