Add "Join room" and "Sign in to start talking" buttons to thread message panel

Add Sign in to start talking button to thread message panel. Currently it's just blank at the bottom when NLI with no indication that you can send a message. This is important for mobile, where the thread panel covers everything else

Add Join room for users who are not members of the room they are looking at.

Current Sign in to start talking in the main message view,

Current Jon room in the main message view,

Relevant code

• https://gitlab.com/gitlab-org/gitter/webapp/blob/f7db1b12ff4692322afede18d70170cf2f9b7916/public/templates/partials/chat_nli_input.hbs#L2-4
• https://gitlab.com/gitlab-org/gitter/webapp/blob/f7db1b12ff4692322afede18d70170cf2f9b7916/public/js/vue/thread-message-feed/components/index.vue#L69

Testing strategy

1. Dark mode