Unable to comment in epics if I'm not a member

Description

Existing permissions from https://docs.gitlab.com/ee/user/permissions.html#group-members-permissions:

• View private group epic: Guest+
• View internal group epic: Guest+
• View public group epic: Guest+ (includes not logged in users which are implicit Guests)
• Edit group epic: Reporter+

Permissions to correct/implement for this issue:

• If you can view an epic (as a logged in user), you should be able to comment on it.
• If you are not logged in, you cannot edit an epic.

Verify UX for cases when you cannot edit comments.

Original description

I mentioned some community members in &622 (closed), and I was surprised to hear that they weren't able to participate in the epic (https://gitlab.com/gitlab-org/gitlab-ce/issues/52092#note_128297214).

I took a look on another account, and noticed this:

1. I'm logged in, but see the "Please register or sign-in to reply" container where the reply to discussion input box should be. Notice that I'm logged in, so none of the presented options would actually solve my problem.
2. I see "This issue is locked. Only project members can comment" at the bottom, where we'd normally present the comment box.
   • It's not clear which project this is referring to, since the epic covers a couple of projects.