Refactor pagination

[ThoWagen]

In this PR the pagination templates are refactored to reduce code duplication and by adding extra classes to some elements.

We want to use those classes to improve the responsive design. On small screens the number of selectable pages shall be reduced and the text on the "Next" and "Prev" buttons removed. If you believe that should be part of the standard VuFind themes, I can also add this here.

TODO

• Update changelog when merging

[demiankatz]

I'm interested in @EreMaijala's opinion on this, since he's refactored some of this code more recently than I have, but it seems pretty reasonable based on my first review. See below for some minor questions and comments.

Regarding whether to include improved responsive design in the core project, I'm certainly open to that, but I think it would make sense to do it as a separate follow-up PR so we don't tie up the basic infrastructure improvements if there end up being extended discussions about details of the aesthetics. :-)

[demiankatz]

Also, just for the record, I ran the full test suite on my end and everything is passing.

[ThoWagen]

@demiankatz I'm definitely open to modify this more but wanted to point out first that changes in Helpers/pagination.phtml are actually the current state in search/pagination.phtml.

[demiankatz]

Since all of my previous observations turned out not to be relevant, I think this is probably ready to go as-is. But I'll wait and see what @EreMaijala has to say before I finalize the merge!