Keywords not split when searching for members

One-sentence description

When searching for firstname lastname, for example, only members with a first name, last name, nickname or username containg exactly the phrase firstname lastname are shown.

Current behaviour

When using multiple keywords in a search query for a member, the index view does not split these keywords. Thus, it only shows members with a field (first name, last name, nickname or username) containing all of the keywords.

See, for example:

Expected behaviour

The search function returns members where each of the keywords is in one of the model's fields.

Steps to reproduce

1. Go to https://thalia.nu/members/
2. Enter multiple keywords that do not belong to the same model field, for example firstname lastname.
3. Hit the search button.