#1178 Create a list of author-specific publishers

[Ahasuerus]

Create a list of author-specific publishers. It should be implemented as a new tab on the Author Bibliography page, similar to the "Summary", "Awards", "Chronological" and "Alphabetical" tabs. It will display a list of publishers which have published a given author's titles (including VTs by pseudonyms) and show how many titles each publisher has published.