reactions: Refactor add_clean_reactions to make_clear_reaction.
reactions: Refactor add_clean_reactions to make_clear_reaction.

This is a pure code refactor for readability.

Previously, we were relying on there being a side effect to
add_clean_reaction which was necessitated by the presence of an output
parameter, `message` (or more specifically `message.clean_reaction`).
Output parameters are confusing.
Hence, this commit changes to have a make_clean_reaction function that
returns a reaction.

kou/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,kou/zulip,andersk/zulip,rht/zulip,kou/zulip,rht/zulip,andersk/zulip,zulip/zulip,kou/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,kou/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,zulip/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip