Check theme overrides when retrieving platform name from settings

lms/templates/emails/activation_email.txt

 <%! from django.utils.translation import ugettext as _ %>
-${_("Thank you for signing up for {platform_name}.").format(platform_name=settings.PLATFORM_NAME)}
+<%! from openedx.core.djangoapps.theming.helpers import get_value as get_themed_value %>
+${_("Thank you for signing up for {platform_name}.").format(
+    platform_name=get_themed_value('PLATFORM_NAME', settings.PLATFORM_NAME)
+)}
 
 ${_("Change your life and start learning today by activating your "
     "{platform_name} account. Click on the link below or copy and "
     "paste it into your browser's address bar.").format(
-      platform_name=settings.PLATFORM_NAME
+        platform_name=get_themed_value('PLATFORM_NAME', settings.PLATFORM_NAME)
     )}
 
 % if is_secure:
@@ -15,4 +18,4 @@ ${_("Change your life and start learning today by activating your "
 ${_("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 "
-      "{platform_name} website.").format(platform_name=settings.PLATFORM_NAME)}
+      "{platform_name} website.").format(platform_name=get_themed_value('PLATFORM_NAME', settings.PLATFORM_NAME))}

lms/templates/emails/activation_email_subject.txt

 <%! from django.utils.translation import ugettext as _ %>
+<%! from openedx.core.djangoapps.theming.helpers import get_value as get_themed_value %>
 
-${_("Activate Your {platform_name} Account").format(platform_name=settings.PLATFORM_NAME)}
+${_("Activate Your {platform_name} Account").format(
+    platform_name=get_themed_value('PLATFORM_NAME', settings.PLATFORM_NAME
+))}

lms/templates/emails/email_change.txt

 <%! from django.utils.translation import ugettext as _ %>
+<%! from openedx.core.djangoapps.theming.helpers import get_value as get_themed_value %>
 ${_("We received a request to change the e-mail associated with your "
     "{platform_name} account from {old_email} to {new_email}. "
     "If this is correct, please confirm your new e-mail address by "
-    "visiting:").format(platform_name=settings.PLATFORM_NAME, old_email=old_email, new_email=new_email)}
+    "visiting:").format(
+        platform_name=get_themed_value('PLATFORM_NAME', settings.PLATFORM_NAME),
+        old_email=old_email,
+        new_email=new_email
+    )
+}
 
 % if is_secure:
  https://${ site }/email_confirm/${ key }
@@ -13,4 +19,4 @@ ${_("We received a request to change the e-mail associated with your "
 ${_("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 "
-  "{platform_name} web site.").format(platform_name=settings.PLATFORM_NAME)}
+  "{platform_name} web site.").format(platform_name=get_themed_value('PLATFORM_NAME', settings.PLATFORM_NAME))}

lms/templates/emails/email_change_subject.txt

 <%! from django.utils.translation import ugettext as _ %>
-${_("Request to change {platform_name} account e-mail").format(platform_name=settings.PLATFORM_NAME)}
+<%! from openedx.core.djangoapps.theming.helpers import get_value as get_themed_value %>
+${_("Request to change {platform_name} account e-mail").format(
+    platform_name=get_themed_value('PLATFORM_NAME', settings.PLATFORM_NAME)
+)}