allow a plugin to override the signup form, refs #CNVS-4198

test plan:
- go to /register_from_website
- the forms should behave normally

Change-Id: I317f4a8404cbf00f2de112a88421c724834e6181
Reviewed-on: https://gerrit.instructure.com/18470
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>
QA-Review: Cam Theriault <cam@instructure.com>

Author: Joe Tanner

app/coffeescripts/registration/signupDialog.coffee

@@ -18,6 +18,7 @@ define [
     return unless templates[id]
     $node = $nodes[id] ?= $('<div />')
     $node.html templates[id](
+      form_url: ENV.FORM_URL
       terms_url: "http://www.instructure.com/terms-of-use"
       privacy_url: "http://www.instructure.com/privacy-policy"
     )

app/controllers/users_controller.rb

@@ -623,6 +623,7 @@ def external_tool
 
   def new
     return redirect_to(root_url) if @current_user
+    js_env[:FORM_URL] ||= '/users' # can be overridden in a plugin
     render :layout => 'bare'
   end
 

app/views/jst/registration/parentDialog.handlebars

@@ -1,5 +1,5 @@
 <div class="registration-dialog">
-  <form class="form-horizontal bootstrap-form" method="post" action="/users">
+  <form class="form-horizontal bootstrap-form" method="post" action="{{form_url}}">
     <div class="control-group">
       <label class="control-label" for="parent_name">{{#t "labels.name"}}Your Name{{/t}}</label>
       <div class="controls">

app/views/jst/registration/teacherDialog.handlebars

@@ -1,5 +1,5 @@
 <div class="registration-dialog">
-  <form class="form-horizontal bootstrap-form" method="post" action="/users">
+  <form class="form-horizontal bootstrap-form" method="post" action="{{form_url}}">
     <div class="control-group">
       <label class="control-label" for="teacher_name">{{#t "labels.name"}}Name{{/t}}</label>
       <div class="controls">