Remove obscure reference to Douglas Adams

Signed-off-by: Thomas Citharel <tcit@tcit.fr>

lib/web/templates/email/group_member_removal.html.heex

@@ -15,7 +15,7 @@
           style="padding: 40px 20px 20px 20px; border-radius: 4px 4px 0px 0px; color: #3A384C; font-family: 'Roboto', Helvetica, Arial, sans-serif; font-size: 48px; font-weight: 400; line-height: 48px;"
         >
           <h1 style="font-size: 48px; font-weight: 400; margin: 0;">
-            <%= gettext("So long, and thanks for the fish!") %>
+            <%= gettext("Until next time!") %>
           </h1>
         </td>
       </tr>

lib/web/templates/email/group_member_removal.text.eex

@@ -1,4 +1,4 @@
-<%= gettext "So long, and thanks for the fish!" %>
+<%= gettext "Until next time!" %>
 ==
 <%= gettext "You have been removed from group %{group}. You will not be able to access this group's private content anymore.", group: @group.name %>
 <%= @group.url %>