pagination arrows a11y update

[StephDriver]

closes #4842
closes #4247

[joemull]

Looks good to me. I found one tiny bug. Also consider combining some of the instances of verbose blocks into single templates which are included in multiple places, to make it easier to maintain going forward.

[joemull]

I think the {{ page }} here is a bug. You caught it in the other paste of this code and removed it. Maybe combine these two instances with a sub-template to make them easier to maintain?

[StephDriver]

...Also consider combining some of the instances of verbose blocks into single templates which are included in multiple places, to make it easier to maintain going forward.

I've had a look into making pagination templates (beyond those which currently exist) but there is a lot of difference between the logic used in each place and each theme needs different classes, all together, this woudl end up with each new template being used once.

However I take the point on standardising - and what can be standardised is the part that the user interacts with, e.g. the previous page and next page buttons could be pulled out as templates. This would also make it easier to update when we get rid of the « etc to replace with an icon set. And it would ensure all page change links are marked up accessibly.

[StephDriver]

page icons:

{% include "elements/icons/page_previous.html" %}

{% include "elements/icons/page_next.html" %}

[ajrbyers]

Great work here.