This change adds a new build dashboard url to the existing appengine app: $dashurl/gccgo/ which will show the build status of gccgo.
* Added Dashboard struct with exported Name, Rel(ative)Path, and Packages fields.
* Added Dashboard Context method that returns an appengine context with a namespace corresponding to the dashboard's name.
* Modified HandlerFuncs to use Dashboard's Context method for all appengine requests.
* Modified ui template to show different title/header for separate dashboard and added dashboard tab.
R=adg
CC=golang-dev
https://golang.org/cl/13753043
