Fix annotation tool infant diseases (#606)

* Don't assume that `lecture` variable is present

For example, when opening a worked example that is not attached
to a particular lecture, it will be `nil`. In this case,
we return the annotation status of the medium itself.

* Don't show emergency link if `lecture` is not present

* Fix wrong tooltip for annotation button

app/models/medium.rb

@@ -1092,7 +1092,7 @@ def subscribed_users
   # if "inherit from lecture" was selected (i.e. if the annotations status of
   # this medium is -1).
   def get_annotations_status # rubocop:todo Naming/AccessorMethodName
-    return lecture.annotations_status if annotations_status == -1
+    return lecture.annotations_status if annotations_status == -1 && lecture.present?
 
     annotations_status
   end

app/views/annotations/_form_content_further_help.html.erb

@@ -1,16 +1,19 @@
 <div id="emergency-link">
 	<% lecture = Medium.find_by_id(@medium_id).lecture %>
-	<% status = lecture.emergency_link_status_for_database %>
-	<% link = lecture.emergency_link %>
-	<% if status == Lecture.emergency_link_statuses[:lecture_link] %>
-	  <% link_name = Lecture.find_by_id(link.tr("^[0-9]", "")).title %>
-	<% elsif status == Lecture.emergency_link_statuses[:direct_link] %>
-    <% link_name = link %>
+  <% if lecture %>
+	  <% status = lecture.emergency_link_status_for_database %>
+    <% link = lecture.emergency_link %>
+    <% if status == Lecture.emergency_link_statuses[:lecture_link] %>
+      <% link_name = Lecture.find_by_id(link.tr("^[0-9]", "")).title %>
+    <% elsif status == Lecture.emergency_link_statuses[:direct_link] %>
+      <% link_name = link %>
+    <% end %>
+
+    <% unless link_name.blank? %>
+      <%= t('admin.annotation.further_help') %> <br>
+      <a href="<%= link %>" target="_blank" rel="noopener noreferrer">
+        <%= link_name %>
+      </a>
+    <% end %>
   <% end %>
-  <% unless link_name.blank? %>
-    <%= t('admin.annotation.further_help') %> <br>
-    <a href="<%= link %>" target="_blank" rel="noopener noreferrer">
-      <%= link_name %>
-    </a>
-	<% end %>
 </div>

app/views/media/play.html.erb

@@ -64,7 +64,8 @@
 
             <!--- ANNOTATION BUTTON -->
             <% if user_signed_in? %>
-              <a id="annotation-button" class="btn" title="<%= t('admin.annotation.edit_annotation') %>">
+              <a id="annotation-button" class="btn"
+                  title="<%= t('admin.annotation.annotation_modal_head_create_annotation') %>">
                 <i class="fas fa-sticky-note" aria-hidden="true"></i>
               </a>
             <% end %>