[Improvement]: Restrict Text fields in DB Models, save empty subscription note as NULL

[Mark90]

Check the DB models for unrestricted Text fields which allow storing an unlimited amount of text. Decide a sensible upper limit on case by case basis.

For SubscriptionTable.note:

• Explicitly make the Text column a nullable String
• On save of the model, check if the note is empty - in that case save it as NULL
• Create migration to update the note field of existing subscriptions, if note contains string of 0 length then store NULL