feat: add client configuration export feature

[rosenbach]

This PR closes #197

It adds an endpoint to export client configurations as prettyprinted json. Furthermore it adds a button on the individual client's view in order to trigger that endpoint.

Screenshot of the change:

[rosenbach]

in the new commit, I added the checkboxes for client selection for a new bulk-export endpoint:

I also did some minor cleaning of the UI: I added the search button as a part of the search field, and exchanged the X for trashcan icons, which I find more fitting
feel free to adapt the PR to your needs

[skoruba]

Thank you! Can you please check conflicts? I want to ask you - can you please run gulp build and added updated scripts and css as well?

[skoruba]

@rosenbach - thanks for you great work, let me test it. 👍

[omon77]

@rosenbach , when basepath for AdminUI is specified, the link generation for ClientsExport action from Search ends up going up one directory.
If my basepath is: localhost:44303/dir/adminui the path for Clients view is: localhost:44303/dir/adminui/configuration/clients, the ClientsExport link created in js is: localhost:44303/dir/adminui/clientsexport resulting in 404.