Add GET parameter for size of the tree list

[padovan]

Feedback from a user:

One thing that would be convenient for me is if there were some way to
set the "Items per page" for my android homepage since the default of
10 is a little too small so I always have to change it to 20. Could we
add a setting that can be appended to the URL to set this?

Maybe an "items=" setting so that instead of my normal top-level page:

https://dashboard.kernelci.org/tree?ts=android

I'd append an "items=20":

https://dashboard.kernelci.org/tree?ts=android&items=20

[padovan]

@joaobertacchi this is for the next sprint