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

Closes #2424 (closed)

Testing was blocked by https://dev.gitlab.org/gitlab/gitter/webapp/merge_requests/48 but now merged ✔

This MR substitutes the chat message input with three different states based on the user who's looking at the thread message feed:

User	The thread input replacement
Logged in user who can join the room
Logged in user who can't join the room
Not logged in user

Reflecting the join request in the component

Testing strategy

• have three rooms with threaded conversations enabled
  • normal public room without a welcome message
  • normal public room with a welcome message
  • GitHub public room that has the "Only GitHub users are allowed to join this room." option enabled
• all the testing is in an open thread in the given room, never in the main message feed
• when the user can join
  • joining a room with welcome message shows the welcome message
  • joining the room without the welcome message is immediate (no welcome message is shown)
• when user can't join
  • visiting the GitHub only room using a GitLab user results in "Only GitHub users can join this room" message
  • have a member of the room mention the user who can't join by sending message to the forbidden room @<testUser>_gitlab (the detailed explanation why we have to test this is here: https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1742#note_276481842)
    • validate that when the GitLab user is online, they can see the correct message
    • validate taht when the GitLab user opens the website after they were mentioned, they can see the correct message
• not logged in user
  • user will see a button "Sign in to start talking"