If an error has occurred while fetching from the API, the
placeholder item should display the API error information.
If the error is specifically a login issue, a specific
error message is displayed.
Also, adjusted the mw.echo.ui.PlaceholderItemWidget to
accept a clickable link; when valid (currently only with
login error) the link is applied so the user can click
the notification and be taken to the login page.
For general notices (like API error or a general 'no
notifications found' message) the link does nothing.
Bug: T121923
Change-Id: I89a43c7c0eb2cf8e63d03704536e0938ab57dd4d
This is in preparation of adding more item models and widget types,
and in preparation of switching the notification widget away from being
a select widget.
Change-Id: I518fb3d80f4f67d677c21ca5593638269acfa544
