appview/pages: hide reaction actions by default
the behavior is like so now:
- comments no longer have a reaction button below them
- like linear/discord/slack: the reaction button shows up on hover in
the comment header to the right
- when existing reactions are present, a reaction button can be seen in
the bottom, *in addition* to the one in the header (just like linear's
comment threads)
Signed-off-by: oppiliappan <me@oppi.li>