[head] Add search octicon to Explore link

Since our Explore page also serves the search function for interfacing
with the indexing service, adding the search octicon (magnifying glass)
makes it clearer for users.

templates/base/head.tmpl

@@ -100,7 +100,7 @@
 									<a class="item{{if .PageIsHome}} active{{end}}" href="{{AppSubURL}}/">{{.i18n.Tr "home"}}</a>
 								{{end}}
 
-								<a class="item{{if .PageIsExplore}} active{{end}}" href="{{AppSubURL}}/explore/repos">{{.i18n.Tr "explore"}}</a>
+								<a class="item{{if .PageIsExplore}} active{{end}}" href="{{AppSubURL}}/explore/repos"><i class="octicon octicon-search"></i>{{.i18n.Tr "explore"}}</a>
 								{{/*<div class="item">
 									<div class="ui icon input">
 									<input class="searchbox" type="text" placeholder="{{.i18n.Tr "search_project"}}">