Dashboard Performance - Store dashboard "Show Variables" setting with each dashboard.

**Proposal:**
In cloud2, I have some dashboards that use variables and some that don't use variables. On the dashboards without variables, I want to recover the screen real estate from the banner that says "This dashboard doesn't have any cells with defined variables. Learn How" and so I hide this banner by unchecking the "Show Variables" toggle. This is great.

BUT when I then switch to a dashboard of mine that does use variables, how the banner/toolbar that let's me change the variables is hidden because the "Show Variables" toggle is global across all my dashboards.

This FR proposal is to make the "Show Variables" setting specific to each dashboard so I can recover screen space when I don't use variables and change variables on the dashboards I regularly change the variables.

**Current behavior:**
Show Variables state is global across all dashboards.

**Desired behavior:**
Show Variables state is local to each dashboard.

**Alternatives considered:**
Current behavior: Click the the Show Variables toggle often.
A keyboard shortcut would help by reducing switching to the mouse, but I think dashboard local setting would be best (along with a keyboard shortcut too!)

**Use case:**

For a more pleasant user experience by reducing user clicking need.

**screenshots**
Banner/toolbar on a dashboard that doesn't use variables and won't use variables

Variables banner hidden saving screen real estate.