Always use UTC Timezone for Silence

When creating a Silence, users can specify the startsAt and endsAt times using a date-time
picker on the UI. This picker displays the current date and time based on the user's browser.
However, these values are sent to the Promgen server without a timezone. As a result, Promgen
automatically adds the server's current timezone as the Silence's timezone. This can lead to
time discrepancies if the timezones between the user and the Promgen server are different.

To fix this issue, we have updated the UI to always use UTC date and time when sending a
Silence creation request to the Promgen server. This ensures that UTC is consistently used
across the client, the server, and Alertmanager.

Author: hoangpn

promgen/static/js/promgen.vue.js

@@ -256,8 +256,12 @@ app.component('silence-create-modal', {
 
             const body = JSON.stringify({
                 matchers: matchers,
-                startsAt: this.form.startsAt,
-                endsAt: this.form.endsAt,
+                startsAt: this.form.startsAt
+                    ? new Date(this.form.startsAt).toISOString()
+                    : undefined,
+                endsAt: this.form.endsAt
+                    ? new Date(this.form.endsAt).toISOString()
+                    : undefined,
                 duration: this.form.duration,
                 createdBy: this.form.createdBy,
                 comment: this.form.comment

promgen/templates/base.html

@@ -66,7 +66,7 @@
   </script>
   <script src="{% static 'js/promgen.js' %}?v=5"></script>
   <script src="{% static 'js/mixins.vue.js' %}"></script>
-  <script src="{% static 'js/promgen.vue.js' %}?v=5"></script>
+  <script src="{% static 'js/promgen.vue.js' %}?v=6"></script>
   {% block javascript %}{% endblock %}
 
   <datalist style="display:none" id="common.labels">