549 enable source removal

Fixes #549. This only allows for source removal if it hasn't been successfully imported (or the loaded data not deleted).

Other changes necessary:
* Dry run for import is now the sample run identifier
* Unique keys are checked in dry runs (sample tests)
* Sample runs are marked as 'sample' operations (imports as 'import') in database
* New workflow for source management
    1. Upload source
    2. Test or delete source
    3. Successfully tested? Load or delete source
    4. Successfully loaded? User cannot do anything
* Deleting loaded data marks all runs of all sources as "removed" (their status)
* Icons and smaller text for buttons (so they better fit the lines)
* Operation buttons are now shown for all source (not only the first one)
* csvimport_fixture (make_account) in tests now gets or creates an account