Add Startup Servlet to Confluence

Problem Definition

When starting Confluence, you will need to wait until it is ready to serve so you can start using it. This process is not transparent to admins/users

Suggested Solution

Jira currently has a startup servlet where it mentions that Jira is starting up. It is very helpful to have a similar startup page for Confluence. Especially on Data Center where you could track the startup of a specific node in the UI rather from the logs.