ensures speaker's multiple talks in a single event display

The performance improvement in #644 to the speaker page implicitly
(and incorrectly) assumes that a speaker will give a single talk
per event. Rather than using .GetPage which can return only a single
page, we can change the original call to .Site.Pages and get the
correct behavior with at least some of the performance improvement.

This could be further improved if the program YAML included the event
name, or if Hugo supported a "contains" or "startswith" operator in
the where function.

Fixes #643 (comment)
Replaces devopsdays-web f944fea71d071b959f715e9cdcb0ebb00f52c978

layouts/speaker/single.html

@@ -16,10 +16,7 @@ <h2 class="speaker-page">{{ .Title }}</h2>
      <h3>{{ .Title }} at {{ $e.city }} {{$e.year}}</h3>
           <ul class="list-group">
             {{- $.Scratch.Set "speaker" .File.BaseFileName -}}
-
-            {{- $p := $.Site.GetPage (printf "events/%s/program/%s" $e.name ($.Scratch.Get "speaker")) -}}
-            {{- if $p -}}
-              {{- with $p -}}
+            {{ range where (where $.Site.Pages "Type" "talk") ".Dir" "=" (print "events/" $e.name "/program/") }}
                 <!-- Now we can display stuff! -->
                 {{- range .Params.speakers -}}
                   {{- if eq . ($.Scratch.Get "speaker") -}}
@@ -30,8 +27,7 @@ <h3>{{ .Title }} at {{ $e.city }} {{$e.year}}</h3>
                   <a href = "{{ .Permalink | absURL }}" class= "list-group-item list-group-item-action">{{ .Title }}</a>
                   {{ $.Scratch.Set "display" "false" }}
                 {{- end -}}
-              {{- end -}} <!-- end with -->
-            {{- end -}} <!-- end if $p -->
+            {{- end -}} <!-- end range where -->
           </ul>
           </div>
   </div>