Fix board pagination

https://www.openproject.org/work_packages/15477

@myabc Would you care to have a look at my impressive Angular skills? :smile_cat: 

What's still missing here is a sync between the OpenProject items-per-page setting and the default the Angular part uses. This results in the number of messages shown not necessarily matching the number of messages shown in the pagination. After one action with the pagination, this is fixed. This might have to do with passing the messages via gon instead of loading them via an API.

Is there any plan on fixing the items-per-page thing or passing it from OpenProject to the Angular part?

Ah, and is there a good way to write a test for this?