[MM-62232] Update loading and welcome screens, theming fixes#3279

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devinbinnie wants to merge 3 commits intomasterfrom MM-62232

+1,858-1,062

Commits on Jan 9, 2025

Replace images with new SVG components, replace backgrounds
devinbinnie
committed
Add Denim and Onyx themes, fix associated themeing issues
devinbinnie
committed

Commits on Jan 10, 2025

Merge branch 'master' into MM-62232
mattermost-build
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