Add "show all" button to groups page

Problem to solve

From !17852 (closed):

When displaying a group page like https://gitlab.com/gitlab-org you can currently only see 20 projects per page. It would be very helpful for us, as we also have a group with many projects, to view them on one page.

Intended users

Everybody who uses groups.

Proposal

Add a Show all button next to the pagination which also persists in the URL.

This is a first step towards infinite scrolling.

(see also !17852 (comment 241505913))