Merge chains in antibody antigen scenario page

[sverhoeven]

Is your feature request related to a problem? Please describe.
At https://www.bonvinlab.org/education/HADDOCK3/HADDOCK3-antibody-antigen/#preparing-the-antibody-structure a pdb file is prepared in a different way than the antibody antigen scenario page does.

Describe the solution you'd like
Instead of just selecting a single chain, have an option to select multiple chains and perform similar preprocessing steps as the cli tutorial.

Describe alternatives you've considered
Perform the prep steps outside the haddock3 webapp.