Add UI action to copy comment contents to clipboard
### Problem to solve
Sometimes it's useful to copy a comment's contents into clipboard to use them in another place.
### Proposal
Add an action to the comment's "More actions" dropdown to copy the comment's content (markdown) to clipboard.
Here's a mockup:
| **Before** | **After** |
| :----: | :-----: |
|  |  |
### Links / references
issue