help: Remove "Invite users" relative help link.

- Replaces `{relative|gear|invite}` with a Markdown include since
  we no longer use the `#invite` hash in the web-app.

help/include/invite-users.md

@@ -0,0 +1,4 @@
+1. Click on the **gear** (<i class="fa fa-cog"></i>) icon in the upper
+   right corner of the web or desktop app.
+
+1. Select <i class="fa fa-user-plus"></i> **Invite users**.

help/invite-new-users.md

@@ -29,7 +29,7 @@ permission to invite users.
 
 {start_tabs}
 
-{relative|gear|invite}
+{!invite-users.md!}
 
 1. Click **Send an email**.
 
@@ -61,7 +61,7 @@ permission to invite users.
 
 {start_tabs}
 
-{relative|gear|invite}
+{!invite-users.md!}
 
 1. Click **Generate invite link**.
 

zerver/lib/markdown/help_relative_links.py

@@ -25,7 +25,6 @@ gear_info = {
     "stats": ["Usage statistics", "/stats"],
     "plans": ["Plans and pricing", "/plans/"],
     "billing": ["Billing", "/billing/"],
-    "invite": ["Invite users", "/#invite"],
     "keyboard-shortcuts": ["Keyboard shortcuts (?)", "/#keyboard-shortcuts"],
     "message-formatting": ["Message formatting", "/#message-formatting"],
     "search-filters": ["Search filters", "/#search-operators"],