PLT-2600/PLT-2770 Added Get Public Link modal and added new API for public file links (#2892)

* Switched public file links to use a GetLinkModal

* Separated getFile and the new getPublicFile api calls

webapp/utils/constants.jsx

@@ -110,6 +110,7 @@ export default {
         TOGGLE_GET_POST_LINK_MODAL: null,
         TOGGLE_GET_TEAM_INVITE_LINK_MODAL: null,
         TOGGLE_REGISTER_APP_MODAL: null,
+        TOGGLE_GET_PUBLIC_LINK_MODAL: null,
 
         SUGGESTION_PRETEXT_CHANGED: null,
         SUGGESTION_RECEIVED_SUGGESTIONS: null,