[4.x] Fix entries on the same date being ignored by collection previous/next tags

[duncanmcclean]

This pull request fixes an issue where the {{ collection:previous }} and {{ collection:next }} tags would ignore other entries published on the same date.

For example, say you had the following entries:

• 1st January: Article A
• 5th February: Article B
• 5th February: Article C
• 7th March: Article D

When you visited the "Article C" entry, the {{ collection:previous }} tag would have returned "Article A" as the previous entry and "Article B" would have been ignored.

Similarly, when you visited the "Article B" entry, the {{ collection:next }} tag would have returned "Article D" as the next entry and "Article C" would have been ignored.

This PR fixes this issue by querying both entries before/after the current entry's date AND those on the same date where the title is greater/less than the current entry's title.

To make this work, ensure you have two sorts configured on the previous/next collection tag. The first being the date field and the second being the secondary column you want to order by, in my case that's the title.

{{ collection:previous in="articles" limit="1" sort="date:asc|title:asc" }}
    <a href="{{ url }}">
        {{ title }} ({{ date format="F j, Y" }})
    </a>
{{ /collection:previous }}

Fixes #1831.