OPAL-1819: Page the variables list

[ymarcon]

Jira issue originally created by user @ymarcon:

Before the server-side search box was introduced, the search was done in the client which therefore needed the whole list of variables in a table. This does not make sense anymore and has a performance price. The variable list should be paged.

[kazoompa]

This is still the case! We should increase the priority of this issue up and paginate the variables via REST.