API access (hidden) to disable moment map server-side saving

[kecnry]

Description

Similar to #2440 (and using the same solution), this pull request allows disabling the support for saving a moment map to a fits file until we change that implementation to save through the browser client-side.

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

Patch coverage is 87.50% of modified lines.

❗ Current head 431f10d differs from pull request most recent head e58cebf. Consider uploading reports for the commit e58cebf to get more accurate results

Files Changed	Coverage
...configs/cubeviz/plugins/moment_maps/moment_maps.py	80.00%
...nfigs/cubeviz/plugins/tests/test_cubeviz_helper.py	100.00%

📢 Thoughts on this report? Let us know!.

[pllim]

Just nitpick on error message. LGTM otherwise. Thanks!

[rosteen]

Looks fine to me, thanks.