(core) New look for site switcher in top-left corner, with support for per-org logos

Summary:
  - Site switcher will show initials (either from user's name or team name),
  - Anonymous users see a grist logo on personal site, but team logo (or initials) on team site,
  - Admin pages (and other pages without orgs) show grist logo,
  - Custom image can be switched on the billing page, common formats are supported up to 100KB.
  - Larger images are down-scaled (on the front-end)
  - SVG larger than 100KB are not accepted
  - Files are stored as data URL's in org prefs,

Test Plan: Added new tests

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D4341

app/client/ui2018/cssVars.ts

@@ -903,6 +903,11 @@ export const theme = {
     colors.mediumGreyOpaque),
   markdownCellMediumBorder: new CustomProp('theme-markdown-cell-medium-border', undefined,
     colors.darkGrey),
+
+  /* App Header */
+  appHeaderBg: new CustomProp('theme-app-header-bg', undefined, colors.light),
+  appHeaderBorder: new CustomProp('theme-app-header-border', undefined, colors.mediumGreyOpaque),
+  appHeaderHoverBg: new CustomProp('theme-app-header-hover-bg', undefined, colors.hover),
 };
 
 const cssColors = values(colors).map(v => v.decl()).join('\n');