Also extend text mail

[ci skip]

app/views/user_mailer/user_signed_up.text.erb

@@ -31,5 +31,14 @@ See docs/COPYRIGHT.rdoc for more details.
 <%= t(:mail_body_register) %>
 <%= @activation_url %>
 
+<%= strip_tags t(:mail_body_register_links_html,
+      webinar_link: OpenProject::Static::Links.links[:webinar_videos][:href],
+      youtube_link: OpenProject::Configuration.youtube_channel,
+      get_started_link: OpenProject::Static::Links.links[:get_started_videos][:href],
+      documentation_link: OpenProject::Static::Links.links[:openproject_docs][:href],
+      contact_us_link: OpenProject::Static::Links.links[:contact_us][:href]
+    )
+%>
+
 <%= t(:mail_body_register_ending) %>
 <%= t(:mail_body_register_closing) %>