Add test notification tool (#28334)

* Add test notification tool

* Add frontend styles

* Remove option from admin view

* Refactor create post and add translations

* Fix several CI errors

* Fix API and frontend snapshots

* Refactor trailing and leading icon on buttons

* Add different button states

* i18n-extract

* Fix wrong text

* Add tests

* Fix wrong string

* Fix test

* feat: E2E send test notifications (#28371)

* Refactor send desktop notification

* Address rest of the feedback

* Fix tests

* Add correct link

* Fix test

---------

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: yasserfaraazkhan <attitude3cena.yf@gmail.com>

api/v4/source/system.yaml

@@ -222,6 +222,30 @@
           $ref: "#/components/responses/Forbidden"
         "500":
           $ref: "#/components/responses/InternalServerError"
+  /api/v4/notifications/test:
+    post:
+      tags:
+        - system
+      summary: Send a test notification
+      description: >
+        Send a test notification to make sure you have your notification settings
+        configured correctly.
+
+        ##### Permissions
+
+        Must be logged in.
+      operationId: TestNotification
+      responses:
+        "200":
+          description: Notification successfully sent
+          content:
+            application/json:
+              schema:
+                $ref: "#/components/schemas/StatusOK"
+        "403":
+          $ref: "#/components/responses/Forbidden"
+        "500":
+          $ref: "#/components/responses/InternalServerError"
   /api/v4/site_url/test:
     post:
       tags: