Use settings in LMS emails, not hardcoded values

Instead of hardcoding things like the platform name, use the
corresponding overrideable settings instead. This allows themes to
control emails as well.

lms/templates/emails/activation_email.txt

-Thank you for signing up for edX! To activate your account, 
-please copy and paste this address into your web browser's
-address bar:
+Thank you for signing up for ${settings.PLATFORM_NAME}! To activate
+your account, please copy and paste this address into your web
+browser's address bar:
 
 % if is_secure:
   https://${ site }/activate/${ key }
@@ -10,4 +10,5 @@ address bar:
 
 If you didn't request this, you don't need to do anything; you won't
 receive any more email from us. Please do not reply to this e-mail; if
-you require assistance, check the help section of the edX web site.
+you require assistance, check the help section of the
+${settings.PLATFORM_NAME} web site.

lms/templates/emails/activation_email_subject.txt

-Your account for edX
+Your account for ${settings.PLATFORM_NAME}

lms/templates/emails/confirm_email_change.txt

 <%! from django.core.urlresolvers import reverse %>
-This is to confirm that you changed the e-mail associated with edX
-from ${old_email} to ${new_email}. If you did not make this request,
-please contact us immediately. Contact information is listed at:
+This is to confirm that you changed the e-mail associated with
+${settings.PLATFORM_NAME} from ${old_email} to ${new_email}. If you
+did not make this request, please contact us immediately. Contact
+information is listed at:
 
 % if is_secure:
   https://${ site }${reverse('contact')}

lms/templates/emails/email_change.txt

-We received a request to change the e-mail associated with your edX
-account from ${old_email} to ${new_email}. If this is correct, please
-confirm your new e-mail address by visiting:
+We received a request to change the e-mail associated with your
+${settings.PLATFORM_NAME} account from ${old_email} to ${new_email}.
+If this is correct, please confirm your new e-mail address by
+visiting:
 
 % if is_secure:
  https://${ site }/email_confirm/${ key }
@@ -10,4 +11,5 @@ confirm your new e-mail address by visiting:
 
 If you didn't request this, you don't need to do anything; you won't
 receive any more email from us. Please do not reply to this e-mail; if
-you require assistance, check the help section of the edX web site.
+you require assistance, check the help section of the
+${settings.PLATFORM_NAME} web site.

lms/templates/emails/email_change_subject.txt

-Request to change edX account e-mail
+Request to change ${settings.PLATFORM_NAME} account e-mail

lms/templates/emails/welcome_body.txt

-edX has launched! To log in, visit:
+${settings.PLATFORM_NAME} has launched! To log in, visit:
 
 % if is_secure:
- https://edx.org
+ https://${settings.SITE_NAME}
 % else:
- http://edx.org
+ http://${settings.SITE_NAME}
 % endif
 
 A login button will be at the top right-hand corner of the window.
@@ -14,7 +14,7 @@ place to reset it.
 
 Thanks for joining us for the ride!
 
-The edX team
+The ${settings.PLATFORM_NAME} team
 
 (Please note that this e-mail address does not receive e-mails --
 if you need assistance, please use the help section of the web

lms/templates/emails/welcome_subject.txt

-Welcome to edX!
+Welcome to ${settings.PLATFORM_NAME}!