Support multi-line tag messages

_Originally opened at !2853 by @clns._

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Tag messages in Git can include a short summary of what changes are included in a release and any other relevant information. So I think GitLab should support multi-line tag messages, both in the "New Tag" interface and when viewing the tag.

Also a non-technical user could use GitLab's interface to create releases, but at the moment this is impossible because he cannot enter a multi-line message. This would've been very handy, otherwise he would need to do it from the command line or other GUI program, which is not something a non-technical user has access to or knows how to do.

This merge request adds support for multi-line tag messages. Below are the impacted screens. Let me know if I should change anything.

This would close #3690 and #2460

![image](/uploads/d33bede1141bfb10c5d9c301ee1cb0ec/image.png)
![image](/uploads/ebfea358c1a5986e7b95ad31748f7a6a/image.png)

See merge request !3833