API to send manual message to users

[mahalakshme]

AC:

• Add an API to send manual message to list of users posted
• POST /api/message
  {
  triggerType: 'manual/automated',
  receiverType: 'Subject/User',
  messageTemplateId: 12345,
  parameters: [@name]
  userIds: [123, 456, 789]
  }
• Should work end-to-end along with Identifying users who aren't actively entering data #825 in sending messages to non-active users
• Can remove anything not needed for end-end functionality done as part of Identifying users who aren't actively entering data #825 like saving of users

Technical details:

• On receiving the API request, store entries in message_request_queue, message_receiver(if already an entry of receiver_id doesn't exist), and manual_message(if already an entry of matching message_template_id, parameters and next_trigger_details doesn't exist)
• No need to create entry in message_rule table.

Out of scope:

Being able to send automated message to subjects/users or manual message to subjects

Testing notes

Should work end-to-end along with #825 in sending messages to non-active users