Change display of token in Datasources panel to a Copy button

[Don-Isdale]

Motivation

The Datasources panel at left shows the primary Pretzel server which the user has logged into, and additional secondary servers, which may be Pretzel / Germinate / BrAPI. The panel shows for each data source the user name and the token received as a result of a successful login; this is used in authorising API requests.
For data administrators and developers, it is also useful when uploading via command line, or sending other API endpoint requests; for users it's better not to show it because it creates visual noise with no benefit.

We can replace it with a Copy button, so that only the user can access it and it is not visible.
In normal layout the token is truncated and so screen-shots are not a significant issue, but it is better practice to omit the token display.