Feature Request: show dashboard variables in cell editor view

__Proposal:__
Show the dashboard variables in the dashboard cell editor view. If a user gets linked to a dashboard cell editor and isn't aware there are variables in play, they may not get any results back.

__Current behavior:__
There is no indicator that there are variables in play, and the user isn't able to change the default one.

__Desired behavior:__
The user can see and change the variables in use.

__Use case:__
I was trying to view a deploy dashboard cell linked to me by a teammate. I was unaware that I needed to change a variable in the dashboard to get the cell data to render correctly. The linkage isn't clear.

The issue does not name files or tests. Start by tracing the dashboard cell editor view and how dashboard variables are loaded and edited; done means variables in use are visible and their values can be changed so linked cells render correctly.

Written by the indexing model from the issue text.