Don't show event history widget for guests.

NOTE: guests still do have access to the relevant API endpoint.
In theory, a persistent and computer literate guest user could
visit the API endpoint directly and retrieve the edit history.
As the edit history may need to be given to users who otherwise
do not have write access, it is considered quite acceptable to
allow guest users to access the endpoint.

Closes #194.

lib/www/client/source/src/views/Log.vue

@@ -340,7 +340,7 @@
                   :href="$route.path+'?label='+encodeURIComponent(label)"
                 >{{label}}</v-chip>
               </span>
-              <dougal-event-edit-history v-if="entry.has_edits"
+              <dougal-event-edit-history v-if="entry.has_edits && writeaccess"
                 :id="entry.id"
                 :disabled="loading"
                 :labels="labels"