Add a "View in GitLab" link in notification emails

When an email notification concerns a specific object (issue, note,
merge request, etc.), add a link to the footer of the email that opens
the item's page in a web browser.

Rationale:

* The link is predictable: always the same text, always at the same
location, like any reliable tool.

* It allows to remove the inline-title in many emails, and leave only
the actual content of the message.