Add rainbow and reverse rainbow colormaps

[pllim]

Description

This pull request is to add requested colormaps to image viewer.

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)? 🐱

[codecov]

Codecov Report

Base: 87.82% // Head: 87.81% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (7d4c5a4) compared to base (1acc87d).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1785      +/-   ##
==========================================
- Coverage   87.82%   87.81%   -0.01%     
==========================================
  Files          95       95              
  Lines       10167    10167              
==========================================
- Hits         8929     8928       -1     
- Misses       1238     1239       +1     

Impacted Files	Coverage Δ
jdaviz/app.py	93.78% <100.00%> (ø)
jdaviz/utils.py	89.38% <0.00%> (-0.89%)	⬇️

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[kecnry]

Don't know why you'd want to, but it does work! 😂

[camipacifici]

Great for velocity fields :)
Looks good to me, thank you!

[kecnry]

I was (mostly) joking - a lot of these colormaps have been... misused... when trying to actually convey information in figures, etc, but they're definitely still useful for visualization/inspection purposes so the more the merrier!

I think this qualifies as trivial, so feel free to merge once the restrictions on main are lifted and conflicts/milestone resolved.

[pllim]

Does Cami's comment count as second approval? 😆

[pllim]

Thanks for the quick reviews!