feat(integration): document alert messages

[nikkothari22]

Send messages to Raven users/channels when a document event happens on any other Frappe app.

For example: send a message when a Purchase Invoice is created.

Features

1. Messages can be in Plain Text, Markdown or HTML with support for Jinja templating. All fields in the document are available to use.
2. Conditions can be set to only send notifications if the document meets them. The condition needs to be an eval script that returns True or False.
3. Set multiple recipients for a notification - either channels or users. Recipients can either be static (for example send this to channel "general"), based on a variable in the document (send the message to the user set in the "expense_approver" field in Expense Claim), or even write Jinja. Jinja helps in cases such as when the document has "Employee ID" but we want to get the user linked to the employee. This could be done using {{ frappe.get_value("Employee", doc.employee, "user_id") }}.
4. Messages are automatically linked to the document hence also appear as document previews on Raven and in the document's timeline.

All notifications are sent in the background and hence do not block regular flow of processes in other apps.

Pending: UI to create/manage notifications from the Raven interface.

Closes #589
Closes #1149