Move continue auth/start mapping button to appear fixed in footer

Pin 'Continue Auth/Start Mapping' button to bottom of page for improved user navigation and simplified OAuth process.

app/views/layouts/_content.html.erb

@@ -15,5 +15,12 @@
         <%= yield %>
       </div>
     </div>
+    <% if content_for? :footer %>
+      <footer class="content-footer bg-body-secondary border-top border-secondary-subtle <%= yield :footer_outer_class %>">
+        <div class="content-inner <%= yield :footer_class %>">
+          <%= yield :footer %>
+        </div>
+      </footer>
+    <% end %>
   <% end %>
 </div>

app/views/site/welcome.html.erb

@@ -77,17 +77,20 @@
 
 <%= render "any_questions" %>
 
-<div class='text-center mb-3'>
-<% if params[:oauth_return_url] %>
-  <a class="btn btn-primary" href="<%= params[:oauth_return_url] %>"><%= t ".continue_authorization" %></a>
-<% else %>
-  <a class="btn btn-primary start-mapping" href="<%= edit_path %>"><%= t ".start_mapping" %></a>
-<% end %>
-</div>
-
 <div class='alert alert-primary'>
   <h2><%= t ".add_a_note.title" %></h2>
   <p><%= t ".add_a_note.para_1" %></p>
   <p><%= t ".add_a_note.para_2_html", :map_link => link_to(t(".add_a_note.the_map"), root_path),
                                       :note_icon => tag.span(:class => "icon note bg-dark rounded-1") %></p>
 </div>
+
+<% content_for :footer_outer_class, "sticky-bottom" %>
+<% content_for :footer do %>
+  <div class='d-flex justify-content-center align-items-center'>
+    <% if params[:oauth_return_url] %>
+      <a class="btn btn-primary" href="<%= params[:oauth_return_url] %>"><%= t ".continue_authorization" %></a>
+    <% else %>
+      <a class="btn btn-primary start-mapping" href="<%= edit_path %>"><%= t ".start_mapping" %></a>
+    <% end %>
+  </div>
+<% end %>