draft: feat: add cluster-wide message

[maxrussell]

WIP, not ready for review, yet!

Ticket

DET-10228 DET-10227 DET-10168 DET-9994

Description

Admin users should be able to set a cluster-wide message that is displayed to all users for things like server maintenance.

Test Plan

Checklist

• [ ] Changes have been manually QA'd
• [ ] User-facing API changes need the "User-facing API Change" label.
• [ ] Release notes should be added as a separate file under docs/release-notes/. See Release Note for details.
• [ ] Licenses should be included for new code which was copied and/or modified from any external code.

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[codecov[bot]]

Codecov Report

Attention: Patch coverage is 37.92049% with 203 lines in your changes missing coverage. Please review.

Project coverage is 48.97%. Comparing base (439734b) to head (4ffeadf). Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #9261      +/-   ##
==========================================
- Coverage   49.00%   48.97%   -0.03%     
==========================================
  Files        1235     1238       +3     
  Lines      160191   160507     +316     
  Branches     2781     2784       +3     
==========================================
+ Hits        78494    78604     +110     
- Misses      81522    81728     +206     
  Partials      175      175              

Flag	Coverage Δ
backend	43.81% <41.55%> (-0.04%)	:arrow_down:
harness	63.85% <25.00%> (-0.11%)	:arrow_down:
web	44.13% <52.45%> (+0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
webui/react/src/components/ClusterMessage.tsx	100.00% <100.00%> (ø)
webui/react/src/stores/determinedInfo.tsx	86.02% <100.00%> (+1.49%)	:arrow_up:
master/internal/db/postgres_clustermessage.go	94.11% <94.11%> (ø)
webui/react/src/App.tsx	0.00% <0.00%> (ø)
master/pkg/model/maintenance_message.go	0.00% <0.00%> (ø)
webui/react/src/components/Navigation.tsx	0.00% <0.00%> (ø)
webui/react/src/services/decoder.ts	20.13% <6.66%> (-0.32%)	:arrow_down:
harness/determined/cli/master.py	23.52% <19.04%> (-2.01%)	:arrow_down:
harness/determined/common/api/bindings.py	40.23% <26.37%> (-0.12%)	:arrow_down:
master/internal/api_master.go	2.38% <0.00%> (-1.30%)	:arrow_down:

... and 5 files with indirect coverage changes