PLT-3466 E10: Add announcement bar feature (#6509)

* E10 - Add announcement bar feature

* Updates per feedback

* Add component tests and snapshots

* Update snapshots

* Updating color picker UI (#6543)

* Add class to body tag when banner is not dismissable and clean up localstorage items when banner changes

* Fixing links (#6544)

* Updating UI for fixed error bar (#6552)

* Truncating text on fixed banner (#6561)

* Plt 3466 - Error bar link states (#6577)

* Updating error bar hover state

* Updating error bar link states

config/config.json

@@ -171,6 +171,13 @@
         "ReportAProblemLink": "https://about.mattermost.com/default-report-a-problem/",
         "SupportEmail": "feedback@mattermost.com"
     },
+    "AnnouncementSettings": {
+        "EnableBanner": false,
+        "BannerText": "",
+        "BannerColor": "#f2a93b",
+        "BannerTextColor": "#333333",
+        "AllowBannerDismissal": true
+    },
     "GitLabSettings": {
         "Enable": false,
         "Secret": "",