Refactor Issue and Candidate Comments

[mhinkie]

Is your feature request related to a problem? Please describe.
Currently Issue comments and candiate comments are saved in different DB-Tables and have different Models in the API although they share the same functionality. These could be merged.

Describe the solution you'd like
Update Comment handling so only one Model (Comment) exists which is used both for Issues and Candiates.

[mhinkie]

For everything containing a Rating, a seperate interface could be introduced (applies also to Evidence-Ratings).

[mhinkie]

Part of this Issue:

BUG: the update function for issue-comments requires the wrong permissions: currently requires ISSUE_EDIT, should require ISSUE_COMMENT or similar (see IssueService).