fix: Check uploaded file size in widget and dashboard (#1975)

* File size check logic in widget

* set maxium file size

* update locale texts

* file size check in dashboard

* dynamincally set file size limit error message

* code climate review fixes

* add alert mixin

* move attahcment bus event to constants

* Move file size check logic to helper

* add specs for file size limit check helper

* changes as per review

app/javascript/dashboard/i18n/locale/en/conversation.json

@@ -58,6 +58,7 @@
     "CHANGE_STATUS": "Conversation status changed",
     "CHANGE_AGENT": "Conversation Assignee changed",
     "CHANGE_TEAM": "Conversation team changed",
+    "FILE_SIZE_LIMIT": "File exceeds the {MAXIMUM_FILE_UPLOAD_SIZE} attachment limit",
     "SENT_BY": "Sent by:",
     "ASSIGNMENT": {
       "SELECT_AGENT": "Select Agent",