Enable multi-device for FormulaResponse

We've already audited this for responsiveness. Just need to turn it on.

JIRA: https://openedx.atlassian.net/browse/MA-1809

common/lib/capa/capa/responsetypes.py

@@ -3151,6 +3151,7 @@ class FormulaResponse(LoncapaResponse):
     allowed_inputfields = ['textline', 'formulaequationinput']
     required_attributes = ['answer', 'samples']
     max_inputfields = 1
+    multi_device_support = True
 
     def __init__(self, *args, **kwargs):
         self.correct_answer = ''