Cannot remove & unindex data

[Enet4]

As one of the parts needed to attain feature parity with the old webapp, there there does not seem to be a way to invoke the data deletion operations at the moment.

We should find a way to fit this one in the new UI. I think the key requirements are:

• it should appear at study, series, and instance level;
• it should be only available to those with the permissions to do so;
• there should be in some kind of division for special actions on an entry (study/series/instance)
• the differences between unindexing and removing should be well emphasized, so that users understand which one they want (if any);
• a confirmation prompt is essential, as it's a lossy and irreversible operation.

[luis-godinho]

I added two buttons for each level one for unindexing and removing, with the proper confirmation prompt. It can be found on the following PR.