Adding a background process executor, and observer.

We will be using the external utilities like pg_dump, pg_dumpall,
pg_restore in background. pgAdmin 4 can be run as a CGI script, hence -
it is not good idea to run those utility in a controlled environment.
The process executor will run them in background, and we will execute
the process executor in detached mode.

Now that - the process executor runs in detached mode, we need an
observer, which will look at the status of the processes. It also reads
output, and error logs on demand.

Thanks - Surinder for helping in some of the UI changes.

web/pgadmin/static/js/alertifyjs/pgadmin.defaults.js

@@ -110,7 +110,7 @@ function(alertify, S) {
         if (contentType.indexOf('text/html') == 0) {
           alertify.notify(
               S(
-                window.pgAdmin.Browser.messages.CLICK_FOR_DETAILED_MSG
+                '%s<br><br>' + window.pgAdmin.Browser.messages.CLICK_FOR_DETAILED_MSG
                ).sprintf(promptmsg).value(), type, 0, function() {
                 alertify.pgIframeDialog().show().set({ frameless: false }).set('pg_msg', msg);
               });