Preferences: Add theme preference to match system theme (#61986)

* user essentials mob! 🔱

lastFile:pkg/api/preferences.go

* user essentials mob! 🔱

* user essentials mob! 🔱

lastFile:packages/grafana-data/src/types/config.ts

* user essentials mob! 🔱

lastFile:public/app/core/services/echo/utils.test.ts

* user essentials mob! 🔱

* user essentials mob! 🔱

lastFile:public/views/index-template.html

* user essentials mob! 🔱

* Restore currentUser.lightTheme for backwards compat

* fix types

* Apply suggestions from code review

Co-authored-by: Christopher Moyer <35463610+chri2547@users.noreply.github.com>

* cleanup

* cleanup

---------

Co-authored-by: Ashley Harrison <ashley.harrison@grafana.com>
Co-authored-by: Joao Silva <joao.silva@grafana.com>
Co-authored-by: Christopher Moyer <35463610+chri2547@users.noreply.github.com>

public/locales/de-DE/grafana.json

@@ -489,7 +489,8 @@
     "theme": {
       "dark-label": "Dunkel",
       "default-label": "Standard",
-      "light-label": "Hell"
+      "light-label": "Hell",
+      "system-label": ""
     },
     "title": "Einstellungen"
   },

public/locales/en-US/grafana.json

@@ -489,7 +489,8 @@
     "theme": {
       "dark-label": "Dark",
       "default-label": "Default",
-      "light-label": "Light"
+      "light-label": "Light",
+      "system-label": "System"
     },
     "title": "Preferences"
   },

public/locales/es-ES/grafana.json

@@ -489,7 +489,8 @@
     "theme": {
       "dark-label": "Oscuro",
       "default-label": "Por defecto",
-      "light-label": "Claro"
+      "light-label": "Claro",
+      "system-label": ""
     },
     "title": "Preferencias"
   },

public/locales/fr-FR/grafana.json

@@ -489,7 +489,8 @@
     "theme": {
       "dark-label": "Sombre",
       "default-label": "Par défaut",
-      "light-label": "Clair"
+      "light-label": "Clair",
+      "system-label": ""
     },
     "title": "Préférences"
   },

public/locales/pseudo-LOCALE/grafana.json

@@ -489,7 +489,8 @@
     "theme": {
       "dark-label": "Đäřĸ",
       "default-label": "Đęƒäūľŧ",
-      "light-label": "Ŀįģĥŧ"
+      "light-label": "Ŀįģĥŧ",
+      "system-label": "Ŝyşŧęm"
     },
     "title": "Přęƒęřęʼnčęş"
   },
@@ -576,4 +577,4 @@
       "option-tooltip": "Cľęäř şęľęčŧįőʼnş"
     }
   }
-}
+}

public/locales/zh-Hans/grafana.json

@@ -489,7 +489,8 @@
     "theme": {
       "dark-label": "深色",
       "default-label": "默认",
-      "light-label": "浅色"
+      "light-label": "浅色",
+      "system-label": ""
     },
     "title": "首选项"
   },