Tag sort is undefined for equal occurrences

[ArthurHoaro]

Currently, linksCountPerTag() returns a list of tags sorted by their number of occurrences across links.

They're sorted using arsort(), which doesn't define the sort behaviour for equal values. This function doesn't produce the same result between PHP 5.6 and 7, hence the tests failure in #1141.

Note:
If two members compare as equal, their relative order in the sorted array is undefined.

Also this comment.

I suggest that we fix that by manually sorting equal tags by alphabetical order.