Unit conversion UI tweaks

[rosteen]

This changes three things that were requested to be updated:

• update the plugin description to be either config-specific or config-generic
• consider renaming flux_or_sb to spectrum_y_type (or similar) and updating UI language
• determine which cases angle unit should be shown (UI and user API). Probably can be disabled in anything without an image viewer.

If "Convert the units of displayed physical quantities." doesn't sound good as more generic language to people I can come up with something else.

[codecov]

Codecov Report

Attention: Patch coverage is 90.47619% with 4 lines in your changes missing coverage. Please review.

Project coverage is 88.42%. Comparing base (3a0aa4c) to head (aba54a9).
Report is 113 commits behind head on main.

Files with missing lines	Patch %	Lines
...configs/cubeviz/plugins/moment_maps/moment_maps.py	77.77%	2 Missing ⚠️
...imviz/plugins/aper_phot_simple/aper_phot_simple.py	88.88%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3178   +/-   ##
=======================================
  Coverage   88.42%   88.42%           
=======================================
  Files         122      122           
  Lines       18286    18286           
=======================================
  Hits        16169    16169           
  Misses       2117     2117           

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[javerbukh]

Looks good!

[haticekaratay]

The diff looks good to me!