webui: introduce support Ukraine theme

[pstorz]

Backport of PR #1117 to bareos-21

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Helpful documentation and best practices

• Git Workflow
• Automatic Sourcecode Formatting
• [Check your commit messages](https://docs.bareos.org/DeveloperGuide/gitworkflow.html#comwebui: introduce support ukraine thememits)

Checklist for the reviewer of the PR (will be processed by the Bareos team)

General

• PR name is meaningful
• Purpose of the PR is understood
• Separate commit for this PR in the CHANGELOG.md, PR number referenced is same
• Commit descriptions are understandable and well formatted
• If backport: add original PR number and target branch at top of this file: Backport of PR#000 to bareos-2x

Source code quality

• Source code changes are understandable
• bareos-check-sources --since-merge does not report any problems
• git status should not report modifications in the source tree after building and testing

[fbergkemper]

Слава України