Polish jira_connect users UI

What does this MR do and why?

• Fix nav bar not rendering theme properly.
• Add avatar to user link.
• Adjust headings, margins and spacing of elements.
• Update margin between text and icon in external_link helper.

Changelog: changed

Related to #352906 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Apply this patch to mock the real flow:

diff --git a/app/controllers/jira_connect/users_controller.rb b/app/controllers/jira_connect/users_controller.rb
index a37c68de299..04f7dae45af 100644
--- a/app/controllers/jira_connect/users_controller.rb
+++ b/app/controllers/jira_connect/users_controller.rb
@@ -8,7 +8,8 @@ class JiraConnect::UsersController < ApplicationController
   before_action :verify_return_to_url, only: [:show]
 
   def show
-    @jira_app_link = params.delete(:return_to)
+    # @jira_app_link = params.delete(:return_to)
+    @jira_app_link = '/'
   end
 
   private

2. Go to http://127.0.0.1:3000/-/jira_connect/users.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.