AA-454 and AA-470: Update language and bug fix for highlights

Since Course Highlights aren't necessarily weekly (self-paced courses),
update the language to be more generic. And then includes a bug fix to
not send highlights to learners after they have unenrolled from a course.

cms/templates/js/course-highlights-enable.underscore

@@ -1,6 +1,6 @@
 <div class="course-highlights-setting">
 <h2 id="highlights-enabled-label" class="status-highlights-enabled-label">
-  <%- gettext('Weekly Highlight Emails') %>
+  <%- gettext('Course Highlight Emails') %>
 </h2>
 <br>
 <% if (highlights_enabled_for_messaging) { %>

cms/templates/js/highlights-enable-editor.underscore

@@ -1,7 +1,7 @@
 <p>
 <%- gettext(
-    'When you enable weekly highlight emails, learners ' +
-    'automatically receive weekly email messages for each section that ' +
+    'When you enable course highlight emails, learners ' +
+    'automatically receive email messages for each section that ' +
     'has highlights. You cannot disable highlights after you start ' +
     'sending them.'
 ) %>
@@ -10,7 +10,7 @@
 <% // xss-lint: disable=underscore-not-escaped %>
 <%= edx.HtmlUtils.interpolateHtml(
     gettext(
-        'Are you sure you want to enable weekly highlight emails? '
+        'Are you sure you want to enable course highlight emails? '
         + '{linkStart}Learn more.{linkEnd}'
     ),
     {