Výběr škol v přihlášce: Work-around na bug v Safari

Safari u <select> negeneruje událost onchange, což je bug.
Doporučený work-around je reagovat také na onblur.

Closes #273.

mo/web/templates/user_join_round.html

@@ -41,7 +41,7 @@
 		</div>
 		<div id='town_list-div' class='form-group row' style='display: none'>
 			<div class='col-sm-offset-2 col-sm-10'>
-				<select id='town_list' name='town_list' onchange='osmo_school.town_picked()'>
+				<select id='town_list' name='town_list' onchange='osmo_school.town_picked()' onblur='osmo_school.town_picked()'>
 				</select>
 			</div>
 		</div>

static/js/osmo.js

@@ -13,6 +13,7 @@ class OSMOSchool {
 		this.prefill_town_query = undefined;
 		this.prefill_town_list = undefined;
 		this.prefill_school = undefined;
+		this.current_town_id = undefined;
 	}
 
 	find_town_error(msg) {
@@ -133,11 +134,14 @@ class OSMOSchool {
 
 	town_picked() {
 		const town_id = document.getElementById('town_list').value;
+		if (town_id != this.current_town_id) {
+			this.current_town_id = town_id;
 			document.getElementById('school-div').style.display = 'none';
 			if (town_id != "") {
 				this.do_get_schools(town_id);
 			}
 		}
+	}
 
 	init() {
 		console.log('OSMO: Init schools');