BUG: Handle incompatible Specviz flux unit

[pllim]

Description

This pull request is to see if we can handle spectrum with incompatible flux unit without having to introduce more links.

While I was at it, I also try the "hold sync" trick on a couple of plotting code, like Maarten said. Technically irrelevant to the bug but doesn't hurt?

Fixes #2459

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

All modified lines are covered by tests ✅

Files	Coverage Δ
jdaviz/app.py	85.21% <100.00%> (+0.03%)	⬆️
jdaviz/configs/specviz/plugins/viewers.py	84.17% <100.00%> (+0.57%)	⬆️
jdaviz/configs/specviz/tests/test_helper.py	100.00% <100.00%> (ø)

📢 Thoughts on this report? Let us know!.

[astrofrog]

Just a couple comments below but otherwise this approach seems fine to me

[pllim]

Thanks for the review, @astrofrog ! I applied your suggestions and added you as co-author for the commit.

[javerbukh]

It's good we no longer silently ignore data with incompatible flux units and make a clear snackbar message that it was not loaded. This seems like the best solution while we work on finding a way to work with /sr units.

[rosteen]

Looks good other than the edit I suggested for the snackbar text. Feel free to merge after accepting that (or rejecting if you feel strongly about it!).