shinyslack 0.0.0.9010

New exported function get_shinyslack_team_id() provides the team id for the Slack workspace used for login.
Breaking: Removed slack_shiny_ui() from exports.
Breaking: The site url is now automatically determined, so site_url is no longer an argument to any function.
Breaking: The "real" ui is no longer automatically wrapped in a cookie handler. Many UIs that use shinyslack won't need to deal with cookies in the normal app, so we shouldn't add extra, unnecessary javascript.
Abstracted UI switcher into {scenes} package.
Added a NEWS.md file to track changes to the package.

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