Removed invalid alerting state from the UI

Update github auth docs

Add AWS Govcloud Region

Reduce alerting states

closes #6488

ref #6444

Removed playlist item when dashboard is deleted

ref #6444

In Github's "allowed organizations" scenario, it makes sense to have
`allow_sign_up` to be `true` as most administrators will expect their
org's github users to be able to sign in without further steps like
creating a local user first.

ux(dashboard): final tweaks to view mode and kiosk mode, better animation for view mode, and kiosk mode can be entered via kiosk url parameter, #6476

fix(ux): fixed issue with row menu gliding over header, or in realting extending outside it's bound and not getttin the menu background

Added OpenTSDB nested templating fix entry to CHANGELOG

Added Slack mention feature

ux(dashboard): lots of tweaks and polish to row menu, added remove X to add panel and row option views, #6442

Restores ability to change constant templates without going into
templating menu