Party selection page: Search for an available sender is performing poorly when there are alot of actors

### Description

We received a TopDesk issue describing that the user was not able to find his desired sender/actor to create an instance for. It is probable that the actor was in fact in the list of actors, but the user was able to find it in the list.
Due to having access to a lot of different senders the pagination and search performed poorly.

Investigate:
- Is it possible to increase the performance of our "display more actors" button?
- Could we make it possible to search not only by sender name, but also org. number?
- Can the pagination be enhanced through a different UX design?

### Additional Information

TopDesk case:
https://github.com/Altinn/altinn-support-private/issues/2792

Start at the party selection page implementation and reproduce the slow actor search and “display more actors” behavior with many senders. Measure the current pagination and search flow, then define done as responsive actor loading, search by sender name and organization number, and a clearer pagination experience.

Written by the indexing model from the issue text.