Participable#participants is called twice when viewing an issue/MR

This method is called twice when viewing an issue or merge request:

1. Once to get the list of participants
2. Once to check if you're subscribed to the Issuable

This results in 2 calls to:

• Get all the documents from Redis
• Parse them to HTML
• Extract the users from them
• Check if the users have access to the current project

Probably the easiest way is to:

1. Move the logic of #participants into #raw_participants
2. Have #participants call #raw_participants and memoize the output in a Hash with the key being the supplied user object (since the participants returned depend on the given user)