make avatar lookup occur at image request (#10540)

speed up page generation by making avatar lookup occur at the browser
not at page generation

* Protect against evil email address ".."

* hash the complete email address

Signed-off-by: Andrew Thornton <art27@cantab.net>

Co-Authored-By: Lauris BH <lauris@nix.lv>

routers/routes/routes.go

@@ -417,6 +417,8 @@ func RegisterRoutes(m *macaron.Macaron) {
 	})
 	// ***** END: User *****
 
+	m.Get("/avatar/:hash", user.AvatarByEmailHash)
+
 	adminReq := context.Toggle(&context.ToggleOptions{SignInRequired: true, AdminRequired: true})
 
 	// ***** START: Admin *****