Add tag's release note content to notification body
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### Description
Integrations are not consistent. For example, if a user integrates `Slack Notifications` and turn on `Issues` for it, after an issue is opened in the project, a message is sent to the Slack channel configured and the message contains:
- the issue name and number in **bold** and
- the description of the issue below it

However, if turning on `Tags` for the same `Slack Notification` integration, only the following is sent to the channel:
`UserX pushed new tag v0.1.0 to group/project`
Both `v0.1.0` and `group/project` in the message above are hyperlinks.
We're currently trying to use tags and leverage the content of [release notes](https://docs.gitlab.com/ce/workflow/releases.html#tags-page-with-button-to-add-or-edit-release-notes-for-existing-git-tag) to broadcast new releases to our slack teams, but the current implementation makes it unfriendly for two reasons:
1. The message doesn't tell you what are the changes or releases associated with the tag (like `Issues`'s description - screenshot above - does)
1. The hyperlink in `v0.1.0` sends the user to `http://gitlab.acme.com/group/project/commits/v0.1.0`
### Proposal
1. The Slack message could contain the release notes' content (when the release note content exists) - in a Slack attachment fashion
1. The hyperlink could point to `http://gitlab.acme.com/group/project/tags/v0.3.0` instead of ``http://gitlab.acme.com/group/project/commits/v0.1.0`
This is also true for `Merge Requests`, not only `Tags`.
### Documentation blurb
While using release notes with tags and having notification-related integrations (e.g. Slack), the notification will contain the release note content
issue