Use Prev/Next pagination for exploring projects

What does this MR do?

This changes the pagination system of the "Explore" page so we don't need to perform expensive COUNT queries.

@tauriedavis @sarrahvesselov This is based on my findings discussed in https://gitlab.com/gitlab-org/gitlab-ce/issues/27390#note_37258599 and the comments following it. Since this affects UI/UX I would greatly appreciate any comments. The style of this pagination is the same as the normal one, except we don't display any page numbers (see screenshot below). On the first/last page the prev/next buttons simply aren't displayed.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/27390