T49 Adds function to remove credentials

[danielgranadoscampos]

• Adds function to remove stored credentials to close Add function to erase credentials #49
• Adds shiny translation to add_token(), current_tickets() and create_tickets()
• Adds new features to add token to close Implement movement across projects in each of the instances #64
• Using a JSON file to store translations instead of a cvs closes Work through shinyi18n warning #80
  Note that:
• Particularly current_tickets() needs further translation Further translate current_tickets() #79
• Code readability could be improved by a new function Add a function to return instance #78
• Translation is still a work in progress as we add more messages and communications, thus certain functions may still print warnings such as:
  Warning messages:
  1: In private$raw_translate(keyword) :
  'Current tickets' translation does not exist.