Add costomized banner message UI

[AllForNothing]

1.Fixes #18719
2.Add Banner Message item to configuration
3.Add banner_message property to systeminfo API

Thank you for contributing to Harbor!

Comprehensive Summary of your change

Issue being fixed

Fixes #(issue)

Please indicate you've done the following:

• Well Written Title and Summary of the PR
• Label the PR as needed. "release-note/ignore-for-release, release-note/new-feature, release-note/update, release-note/enhancement, release-note/community, release-note/breaking-change, release-note/docs, release-note/infra, release-note/deprecation"
• Accepted the DCO. Commits without the DCO will delay acceptance.
• Made sure tests are passing and test coverage is added if needed.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed in website repository.

[codecov]

Codecov Report

Merging #18827 (4cccd73) into main (8fe5618) will increase coverage by 0.01%.
The diff coverage is 47.67%.

@@            Coverage Diff             @@
##             main   #18827      +/-   ##
==========================================
+ Coverage   67.38%   67.40%   +0.01%     
==========================================
  Files         981      981              
  Lines      107072   107188     +116     
  Branches     2678     2698      +20     
==========================================
+ Hits        72149    72248      +99     
- Misses      31043    31057      +14     
- Partials     3880     3883       +3     

Flag	Coverage Δ
unittests	67.40% <47.67%> (+0.01%)	⬆️

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

Impacted Files	Coverage Δ
src/lib/config/userconfig.go	68.62% <0.00%> (-0.91%)	⬇️
src/server/v2.0/handler/systeminfo.go	0.00% <0.00%> (ø)
...ell/app-level-alerts/app-level-alerts.component.ts	41.11% <4.34%> (-12.63%)	⬇️
src/controller/systeminfo/controller.go	60.29% <42.85%> (-2.01%)	⬇️
...ide-nav/config/system/system-settings.component.ts	54.65% <60.00%> (+1.58%)	⬆️
...rc/app/base/left-side-nav/config/config.service.ts	57.57% <100.00%> (+2.73%)	⬆️
...portal/src/app/base/left-side-nav/config/config.ts	100.00% <100.00%> (ø)
src/portal/src/app/services/app-config.ts	100.00% <100.00%> (ø)
...al/src/app/services/event-service/event.service.ts	78.37% <100.00%> (+0.60%)	⬆️

... and 14 files with indirect coverage changes

[chlins]

lgtm