Improve the Reliability of Application Links In a Reverse Proxy Environment

The current Application Links mechanism is plagued by a number of issues, when it comes to integration between Confluence and JIRA in a reverse proxy environment. Examples include:

https://jira.atlassian.com/browse/CONF-36648
https://jira.atlassian.com/browse/CONF-33654
https://jira.atlassian.com/browse/CONF-22283

This is especially a problem when the following condition is not met:
(taken from Configuring Tomcat for use with a HTTP proxy ):

When using a reverse proxy, the application server (Tomcat) must be aware of the proxy to ensure that the correct addresses and URLs are sent back to the client. If this is not correctly set up, Tomcat will return the hostname and IP that it's listening on, rather than the address that clients use to access the application.

Different reverse proxies have different configurations making it difficult for our customers to determine the optimal configuration to make Tomcat "aware" of the proxy. We should make this requirement as relaxed as possible, to avoid NPE such as the following:

java.lang.NullPointerException
	at com.atlassian.confluence.plugins.jira.AppLinksProxyRequestServlet.doProxy(AppLinksProxyRequestServlet.java:73)
	at com.atlassian.confluence.plugins.jira.AbstractProxyServlet.doGet(AbstractProxyServlet.java:46)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:620)
	at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)