Make the "Invite Administrators" selection style match the rest of the application

[NicolasDontigny]

We need to make sure this dropdown matches the style in the rest of the application.
This can be done either of 2 ways:
a) Keep the MUI component, but adjust the style to match other dropdowns, if we want to keep the pills
b) Just switch to the bootstrap dropdown with autocomplete, displaying the selected sites below the dropdown, similar to selectors in the Batch Form

We can ask Yili to confirm which UI would be preferred for the selection, either pills inside the input or a list displayed below it.