[UI, ADMIN] Improve UI of /admin/users

[Felienne]

Idea incl level The admin interface (that only I use at the moment) could use a bit of love. I am grouping all issues here, if someone wants to pick up one, feel free to split out an issue!!

Firstly, this is what it looks like for me (not sure many people use it locally):

1. Filtering (general)

It is not possible to search for all fields, just a few. Would be nice to have them all, esp. the "is teacher" field/column, so I can ok them at once:

2. Filtering and pagination

We recently added pagination, but search seaches per page, so if I search for an old user, I have to click next next next. Not ideal.

3. Filtering on non-English keywords

I can search for users of 1 keyword language, but I am also interested to see all non-English users (maybe we'd want such an overview for the /admin/stats too)

4. Decline teacher request

Right now I get a question mark for a request, if I click it, it get's oked. But sometimes a kid clicks the button and I want to decline. That now is only possible with 2 clicks: accept and then delete again. Not a problem for me to click twice but it also means the kid gets an email that they teacher's account is approved which is annoying.