Allow pinning dashboards and panels to front, and prevent closing while pinned

[dsmmcken]

As a user I want to mark certain important panels and dashboards important and not closeable, to avoid accidently closing them.

Spec: Similar to pinning a browser tab, a pinned dashboard or pinned table should gain a pin icon, and not have a close icon while pinned (nor reserve the UI space for a close icon). Dashboard tabs, and panels should be pinnable/un-pinnable from their context menu. When pinned move the item to the start of the stack dashboard stack, or panel stack.

Pinned dashboard:

Pinned panel:

Insert a context menu labeled as "Pin tab"/ "Unpin tab" as it's own group above the "close" group in the respective menus.