R1:58f66fea804e

Allow users to remove their own comments, and administrators to remove any comment

Summary:
Fixes T4909. Adds a "remove" link next to the edit link, which permanently hides a comment. Addresses two use cases:

- Allowing administrators to clean up spam.
- Allowing users to try to put the genie back in the bottle if they post passwords or sensitive links, etc.

The user who removed the comment is named in the removal text to enforce some level of administrative accountability.

No data is deleted, but…