Add support for variables in Dashboard Cell titles

__Proposal:__
Dashboard cells can be named with units and attributes that change. An example of this is a cell with data that performs time groupings (i.e., `aggregateWindow`). I (and I'm sure other users) would like for the cell to tell me and any other user what time grouping the cell is looking at.

A cell title of `Requests per host per ` is more meaningful than just `Requests per host`.

__Current behavior:__
If this information is needed, the time unit either has to be hard coded or the user has to read the timestamps of data values and get the difference on their own; manually read the interval provided by the UI. This can be a pain to do frequently, exacerbated by certain timestamp formats. One could rename cells manually but they'd have to do it on every cell that calls for it....after reading what the timestamps again in order to determine the time window the UI provided.

No file, test, or entry point is named in the issue. First clarify which dashboard title variables and changing attributes are supported, then trace the dashboard cell-title handling and define completion as titles updating to show the selected time grouping without manual renaming.

Written by the indexing model from the issue text.