Allow configuring max number of users per page in admin

[yuvipanda]

Proposed change

Right now, it's impossible to have more than 250 users per-page in the JupyterHub admin page. However, this makes searching for a single user basically impossible - you have to manually paginate through them.

Search functionality is the right way to do this, but until then, I think we should allow using a traitlet to configure how many users can be displayed on a single page.

Alternative options

Add a user search bar! But that's more work.

Who would use this feature?

Anyone with more than 250 users on their hub.

(Optional): Suggest a solution

Add a traitlet to the JupyterHub object that gets passed down to the Pagination class, where it can use those numbers instead of the hardcoded limits there.

[minrk]

#3229 adds configuration c.Pagination.max_per_page = 1000

[yuvipanda]

The fix in #3229 doesn't seem to work, and setting pagination has no effect.

        c.Pagination.default_per_page = 10000
        c.Pagination.max_per_page = 10000

I've currently fixed this by hardcoding values in a fork (yuvipanda@a89ce8f), but should be fixed properly.

[jbaksta]

The pagination seems to work for us, but with a large caveat that we don't see all the records for users. The UI claims the proper count of users, but when it integrates named servers, we're missing a slew of user records at the end (default, we lose username records that start with 'o'). I'll admit to not have gone digging down into this just yet, but I'm confident the UI is presenting me with 496 records for a count of users only. The users from the API shows 496, but if I take the named servers into account, even just one, there should be more than 496 records to display. FWIW, reverse sorting will show the issue the other way.

[manics]

The admin pages have had a big overhaul. Is this problem still present?

[jbaksta]

I would say with the new search feature implemented has alleviated this issue, IMO anyway. I think there are certainly tradeoffs that happened, but yes the search is extremely useful.