Free Text variable for Dashboard

__Proposal:__
Variable that doesn't have any values populated in the filter. The user can type a value or copy/paste a value to the text field. The filter keeps the recent values used.

__Current behavior:__
Feature doesn't exist. Only Map, Query or CSV can be choosen.

__Desired behavior:__
User should be able to enter a free value for the variable in the dashboard. A default value should be able to be defined in the settings and also the value type e.g. integer, string...

__Alternatives considered:__
No smart alternative exists.

__Use case:__
More flexibility if a e.g. constant changes. Microsoft Azure already offers this feature and it helps e.g. to limit the presented values.

The issue identifies dashboard variable filters but names no files or tests. Start by locating the dashboard variable settings and the existing Map, Query, and CSV variable entry points. Done means users can enter and retain free values, configure a default and value type, and use the value in the dashboard.

Written by the indexing model from the issue text.