SlackUser: use profile.display_name if available
SlackUser: use profile.display_name if available

Since recent changes in Slack, usernames can no longer be set directly, and
are instead inferred from the email address. This is not particularly useful,
so the recommendation is to instead use display_name. display_name may have
spaces in it, however, which confuses tab completion.

With this change, wee-slack uses display name for both display and completion,
after first stripping all whitespace from it. Ref linking (via linkify_text)
ensures that @mentions continue to work properly even when this whitespace
removal means that the name being @mentioned locally doesn't match the "real"
username.

rawdigits/wee-slack,wee-slack/wee-slack,trygveaa/wee-slack