Adds EchoEventPresentationModel::canRender() for notification types to
indicate that something can't be rendered if for example, a page is
deleted.
In that case, the notification is marked as read in a deferred update.
All callers were also updated to check if the notification was formatted
properly.
Bug: T116888
Change-Id: Idb975feaec893ef86c41cc487102e3539c07e328
The workflow to format a notification is
* Get EchoEvent, User, and Language
* Get EchoEventFormatter implementation for notification type
** EchoEventFormatter returns structured data about each part of the
notification (header, body, primary link, secondary link(s))
* Each output type will have a formatter class (e.g.
EchoSpecialNotificationsFormatter, EchoPlainTextEmailFormatter) which
takes a EchoEventPresentationModel and generates whatever it wants
(HTML, plain-text email, etc).
Included is an example conversion of the user-rights and mention
formatters. The previous infrastructure will remain in place for
backwards compatability until other extensions can be updated.
Bug: T107823
Change-Id: I4397872a7ec062148dfcb066ddd8ab83f40486ac
