CRT: @thread option for the autocomplete on a thread

When Collapsed Reply Threads are enabled, @thread would cause a mention and auto-follow the thread for every participant, including the root post author.

This should only appear in the autocomplete of the RHS or Thread Viewer. If @thread is typed out in the center channel it should render as plain text and not appear in the autocomplete

Also add "thread" to the list of restricted usernames

Note: This needs to be implemented on both webapp and mobile

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JIRA: https://mattermost.atlassian.net/browse/MM-37170