[auth] Add Postmark logo (#5611)

## Description
Adds a logo for Postmark (https://postmarkapp.com/).

Postmarkapp doesn't have any official branding source files or use SVGs,
so Vector logo is sourced from here:
https://www.vectorlogo.zone/logos/postmarkapp/
![Postmarkapp vector from VectorLogo
Zone](https://www.vectorlogo.zone/logos/postmarkapp/postmarkapp-icon.svg)

I went with this rather than the ones on sites like
https://brandfetch.com/postmarkapp.com because the white stamp border
looks the closest to the logo that Postmark uses in their official
Zapier integration.
https://zapier.com/apps/postmark/integrations

![Postmarkapp Zapier
Logo](https://zapier-images.imgix.net/storage/services/e1cd4dba89396528988daf12ba63d35d.png?auto=format&fit=crop&q=98&w=32&h=32&dpr=3&bg=black)

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

@@ -905,6 +905,13 @@
     {
       "title": "PostNL"
     },
+    {
+      "title": "Postmark",
+      "slug": "postmarkapp",
+      "altNames": [
+        "postmarkapp"
+      ]
+    },
     {
       "title": "PostScan Mail",
       "slug": "postscanmail",

auth/assets/custom-icons/icons/postmarkapp.svg

@@ -0,0 +1 @@
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