Support customisation of search configuration for each microsite

[finnlewis]

@nccchris raised an interesting question in our Technical Drop-in today:

Q. How can we allow the site-wide saerch of each microiste to be configured differently? We want to exclude venues (or other content types) from the search on a specific microsite.

A. For one microsite, we should be able to create a new index, a new search_api view and deploy that on a different path (not the /search)

But in general, this might be a good feature to research, define and build, if it becomes a popular requirement.

[finnlewis]

@karlwist0 (Sheffield) has 3 microsites and might need something like this in the future.