Implements keyword sub feature for bulk emails

This commit pulls in changes from #4487 that implements keyword
substitution for bulk emails. With these changes, an instructor can
include keywords in their bulk emails which will be automatically substituted
with the corresponding value for the recepient of the email. Keywords are
of the form %%keyword%%, and the keywords implemented in this commit include:

%%USER_ID%% => anonymous_user_id
%%USER_FULLNAME%% => user profile name
%%COURSE_DISPLAY_NAME%% => display name of the course
%%COURSE_END_DATE%% => end date of the course

Client-side validations have also been implemented to ensure that only emails
with well-formed keywords can be sent.
The architecture is designed such that adding in new keywords in the future
would be relatively straight-forward.

lms/static/coffee/src/instructor_dashboard/send_email.coffee

@@ -13,6 +13,7 @@ PendingInstructorTasks = -> window.InstructorDashboard.util.PendingInstructorTas
 create_task_list_table = -> window.InstructorDashboard.util.create_task_list_table.apply this, arguments
 create_email_content_table = -> window.InstructorDashboard.util.create_email_content_table.apply this, arguments
 create_email_message_views = -> window.InstructorDashboard.util.create_email_message_views.apply this, arguments
+KeywordValidator = -> window.InstructorDashboard.util.KeywordValidator
 
 class SendEmail
   constructor: (@$container) ->
@@ -42,6 +43,14 @@ class SendEmail
         alert gettext("Your message cannot be blank.")
 
       else
+        # Validation for keyword substitution
+        validation = KeywordValidator().validate_string @$emailEditor.save()['data']
+        if not validation.is_valid
+          message = gettext("There are invalid keywords in your email. Please check the following keywords and try again:")
+          message += "\n" + validation.invalid_keywords.join('\n')
+          alert message
+          return
+
         success_message = gettext("Your email was successfully queued for sending.")
         send_to = @$send_to.val().toLowerCase()
         if send_to == "myself"

lms/static/coffee/src/instructor_dashboard/util.coffee

@@ -300,6 +300,31 @@ class PendingInstructorTasks
       error: std_ajax_err => console.error "Error finding pending instructor tasks to display"
     ### /Pending Instructor Tasks Section ####
 
+class KeywordValidator
+
+    @keyword_regex = /%%+[^%]+%%/g
+    @keywords = ['%%USER_ID%%', '%%USER_FULLNAME%%', '%%COURSE_DISPLAY_NAME%%', '%%COURSE_END_DATE%%']
+
+    @validate_string: (string) =>
+      regex_match = string.match(@keyword_regex)
+      found_keywords = if regex_match == null then [] else regex_match
+      invalid_keywords = []
+      is_valid = true
+      keywords = @keywords
+
+      for found_keyword in found_keywords
+        do (found_keyword) ->
+          if found_keyword not in keywords
+            invalid_keywords.push found_keyword
+      
+      if invalid_keywords.length != 0
+        is_valid = false
+      
+      return {
+        is_valid: is_valid,
+        invalid_keywords: invalid_keywords
+      }
+
 # export for use
 # create parent namespaces if they do not already exist.
 # abort if underscore can not be found.
@@ -314,3 +339,4 @@ if _?
     create_email_content_table: create_email_content_table
     create_email_message_views: create_email_message_views
     PendingInstructorTasks: PendingInstructorTasks
+    KeywordValidator: KeywordValidator