settings: Add missing class to labels in stream settings.

Some labels in stream settings were missing the class name for settings
labels. Add the missing class name to the labels.

Fixes part of zulip#21769.

web/templates/stream_settings/stream_types.hbs

@@ -54,7 +54,7 @@
     <div class="advanced-configurations-collapase-view hide">
 
         <div class="input-group">
-            <label class="dropdown-title" for="{{prefix}}stream_post_policy">{{t 'Who can post to this channel'}}
+            <label class="dropdown-title settings-field-label" for="{{prefix}}stream_post_policy">{{t 'Who can post to this channel'}}
                 {{> ../help_link_widget link="/help/stream-sending-policy" }}
             </label>
             <select name="stream-post-policy" class="stream_post_policy_setting prop-element settings_select bootstrap-focus-style" id="{{prefix}}stream_post_policy" data-setting-widget-type="number">
@@ -74,7 +74,7 @@
         {{#if (or is_owner is_stream_edit)}}
             <div>
                 <div class="input-group inline-block message-retention-setting-group time-limit-setting">
-                    <label class="dropdown-title" for="{{prefix}}message_retention_days">{{t "Message retention period" }}
+                    <label class="dropdown-title settings-field-label" for="{{prefix}}message_retention_days">{{t "Message retention period" }}
                         {{> ../help_link_widget link="/help/message-retention-policy" }}
                     </label>