Export source selection in EE

https://redmine.teklia.com/issues/5736

Requires backend#1705 (closed), enterprise#2

The exports Pinia store should include a method that calls ListExportSources and stores its results.

When the enterprise feature flag is enabled, the exports modal should call this new method, and list the available sources to the user so they can select one before starting an export.

This is probably best done with a simple <select> and not a modalception, since it means less clicks for users. The select options should include the last update date when it is set, so that users can tell how obsolete a source is.