Show "suggest edit" and "flag" links when not signed in, and prompt for signin if clicked

[cellio]

meta:291586

Some elements, such as the tag "subscribe" button, are present for everybody but if you're not logged in, clicking them prompts you to log in. Can we do that on posts for "suggest edit" and "flag"? Two reasons:

1. visibility: the new user can see that yes there's a way to alert us to that offensive content or fix that post's formatting
2. convenience: you don't have to go to the top of the page, sign in, find your way back to possibly an answer far down on the page, and then do the thing (the reason pointed out in the meta post)

"Suggest edit" is a link so I assume we could do it the same way as for other links that require sign-in. "Flag" is a modal, so I don't know if we can intercept that to prompt for login -- that part of this request might not be practical. I guess we'd have to show a flag link when not signed in and turn it into the flag modal after sign-in?