Change search and filter icons to SVG (#13473)

Unfortunately, the Fomantic CSS relies on i.icon instead of .icon so a
wrapper is necessary for these with an accompanying class to center the
icon.

templates/repo/search.tmpl

@@ -7,7 +7,7 @@
 				<div class="ui fluid action input">
 					<input name="q" value="{{.Keyword}}" placeholder="{{.i18n.Tr "repo.search.search_repo"}}">
 					<button class="ui button" type="submit">
-						<i class="search icon"></i>
+						<i class="icon df ac jc">{{svg "octicon-search" 16}}</i>
 					</button>
 				</div>
 			</form>