[PLT-6996] On "About Mattermost", add notes with links to notice.txt which has list of copyrights of open source components used (#7315)

* on "About Mattermost", add notes with links to notice.txt which has list of copyrights of open source components used

* remove apps from link

* Minor UI update

webapp/tests/components/__snapshots__/about_build_modal.test.jsx.snap

@@ -141,7 +141,18 @@ exports[`components/AboutBuildModal should match snapshot for enterprise edition
       </div>
     </div>
     <div
-      className="about-modal__hash form-group padding-top x2"
+      className="about-modal__notice form-group padding-top x2"
+    >
+      <p>
+        <FormattedHTMLMessage
+          defaultMessage="Mattermost is made possible by the open source software used in our <a href=\\"https://about.mattermost.com/platform-notice-txt/\\" target=\\"_blank\\">platform</a>, <a href=\\"https://about.mattermost.com/desktop-notice-txt/\\" target=\\"_blank\\">desktop</a> and <a href=\\"https://about.mattermost.com/mobile-notice-txt/\\" target=\\"_blank\\">mobile</a> apps."
+          id="about.notice"
+          values={Object {}}
+        />
+      </p>
+    </div>
+    <div
+      className="about-modal__hash"
     >
       <p>
         <FormattedMessage
@@ -315,7 +326,18 @@ exports[`components/AboutBuildModal should match snapshot for team edition 1`] =
       </div>
     </div>
     <div
-      className="about-modal__hash form-group padding-top x2"
+      className="about-modal__notice form-group padding-top x2"
+    >
+      <p>
+        <FormattedHTMLMessage
+          defaultMessage="Mattermost is made possible by the open source software used in our <a href=\\"https://about.mattermost.com/platform-notice-txt/\\" target=\\"_blank\\">platform</a>, <a href=\\"https://about.mattermost.com/desktop-notice-txt/\\" target=\\"_blank\\">desktop</a> and <a href=\\"https://about.mattermost.com/mobile-notice-txt/\\" target=\\"_blank\\">mobile</a> apps."
+          id="about.notice"
+          values={Object {}}
+        />
+      </p>
+    </div>
+    <div
+      className="about-modal__hash"
     >
       <p>
         <FormattedMessage