Allow to manage the blacklist from Hatstall

The goal of this task is to give access to manage the blacklist from the Hatstall interface.

The rationale for this is that when dealing with wrongly assigned merges and these are huge, that's the point where I check invalid email addresses or names and produce a list of blacklist identities.

Then the next step would consist of adding the new blacklisted identities so they're ignored by SortingHat in the following merge processes.

No file or test is identified in the issue. Start by reviewing Hatstall's existing interface and the SortingHat blacklist flow, then determine where blacklist identities are managed. Done means users can add and manage blacklisted identities in Hatstall and those identities are ignored during subsequent merge processes.

Written by the indexing model from the issue text.