(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/common/themes/GristDark.ts

@@ -568,4 +568,9 @@ export const GristDark: ThemeColors = {
   'markdown-cell-light-bg': '#494958',
   'markdown-cell-light-border': '#32323F',
   'markdown-cell-medium-border': '#555563',
+
+  /* App Header */
+  'app-header-bg': '#32323F',
+  'app-header-border': '#FFFFFF00',
+  'app-header-hover-bg': 'rgba(111,111,125,0.6)',
 };

app/common/themes/GristLight.ts

@@ -568,4 +568,9 @@ export const GristLight: ThemeColors = {
   'markdown-cell-light-bg': '#F7F7F7',
   'markdown-cell-light-border': '#E8E8E8',
   'markdown-cell-medium-border': '#D9D9D9',
+
+  /* App header */
+  'app-header-bg': 'var(--grist-theme-page-panels-main-panel-bg)',
+  'app-header-border': 'var(--grist-theme-menu-border)',
+  'app-header-hover-bg': 'var(--grist-theme-hover)',
 };