[auth] add custom icon for Startmail (#6611)

Adding Custom Icon for Startmail.com

## Description
Add custom SVG icon for [Startmail](https://www.startmail.com/) to
support branding in UI components.
## Tests

mobile/apps/auth/assets/custom-icons/_data/custom-icons.json

@@ -1295,6 +1295,10 @@
         "PAYDAY 3"
       ]
     },
+    {
+      "title": "Startmail",
+      "slug": "startmail"
+    },
     {
       "title": "STRATO",
       "hex": "FF8800"

mobile/apps/auth/assets/custom-icons/icons/startmail.svg

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Generator: Adobe Illustrator 27.0.1, SVG Export Plug-In . SVG Version: 6.00 Build 0)  -->
+<svg version="1.1" id="Слой_1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" x="0px" y="0px"
+	 width="500px" height="500px" viewBox="0 0 500 500" style="enable-background:new 0 0 500 500;" xml:space="preserve">
+<style type="text/css">
+	.st0{fill:#6573FF;}
+	.st1{fill:#202945;}
+</style>
+<g>
+	<path class="st0" d="M500,47.2C500,20.9,478.6,0,452.9,0H47.7C21.4-0.5,0,20.9,0,47.2v43.9c0,0,186.4,180.6,250.5,180.6
+		C319.6,271.7,500,92.2,500,92.2S500,56.5,500,47.2z"/>
+	<path class="st1" d="M0,452.8C0,479.1,21.4,500,47.2,500h405.6c26.3,0,47.2-21.4,47.2-47.2V142.7c0,0-159.2,184.4-249.7,184.4
+		C160.8,327.1,0,178.4,0,178.4C0,236.6,0,395.2,0,452.8z"/>
+</g>
+</svg>