Handling Contributors is painfully slow in the backend, impacting on publishers' workflows

[tosteiner]

Describe the bug
Am noticing considerable lag when calling up the Contributors dialogue (mainly within the book- and chapter-level lookup of contributors).

So, for example, when I'm seeking to add a new contributor to a given work, the dialogue freezes for ca. 5 seconds, and my browser (Firefox) issues a warning that "this site is slowing down your system".

Francois from African Minds also noted they're having similar issues, so would be good to find a fix for this to improve contributor data loading

[ja573]

We should disable updating the base contributor, and only query the one selected