portico: Improve layout of /go page.

Made the input field of the page consistent with
the other similar pages. Here are I have done the following things:
1. Changed text 'Enter your organization's Zulip URL:'->'Organization URL'.
2. Left aligned the label and button to make it consistent.
3. Moved 'Don't know your organization URL? Find your organization.' to be
just below the URL field.
4. Changed the placeholder 'your-organization-url' -> 'your-organization'

Fixes #32198

tools/lib/capitalization.py

@@ -128,7 +128,7 @@ IGNORED_PHRASES = [
     # Emoji name placeholder
     r"leafy green vegetable",
     # Subdomain placeholder
-    r"your-organization-url",
+    r"your-organization",
     # Used in invite modal
     r"or",
     # Units