More linkifier fixes
More linkifier fixes

 * Accept underscores in usernames
 * Accept usernames & room aliases on localhost
 * Accept #localhost:foo.com and @localhost:foo.com

matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk,matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk,aperezdc/matrix-react-sdk,matrix-org/matrix-react-sdk,aperezdc/matrix-react-sdk