Add customizable greeting to index page

Allow an organization to copy the greeting.html file out of the themes
directory, customize it, then point the greeting.file config option at
it so that it will survive Inbucket upgrades.

themes/integral/templates/root/index.html

@@ -9,11 +9,4 @@
 </div>
 {{end}}
 
-{{define "content"}}
-<p>Inbucket is an email testing service; it will accept email for any email
-address and make it available to view without a password.</p>
-
-<p>To view email for a particular address, enter the username portion
-of the address into the box on the upper right and click <em>go</em>.</p>
-{{end}}
-
+{{define "content"}}{{.greeting}}{{end}}