Collaborators can delete Issue Comments

[PaulAdams-Conti]

Issue

Impacted version: 4.28.0

Deployment mode: Tomcat 7

Problem description:
Collaborators can delete any Issue Comment (maybe related to #1462). IMO, only repo admins should be able to delete comments otherwise issue traceability is compromised.

Also unlike in GitHub, there is no record that anyone has deleted a comment, it just disappears.

It would be good if there was some kind of history for the comments so that if necessary, deleted comments can be retrieved in some way.

So my prioritised feature request is as follows:

1. Only repo admins can delete comments.
2. If a comment is deleted, a message should be shown inline with the comments that the comment was deleted and by whom (like in GitHub).
3. Some retrieval possibility or history log for deleted comments at least for repo admins.