Pagination on lessons pages is not consistent.

[MaximilianoRicoTabo]

• Filter navigation sql query to get next post and previous post by menu_order

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Make sure the code does what it says - done -
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Does the code follow the way we’ve done it before? - done -

Changes proposed in this Pull Request:

Hook into get_next_post_where, get_previous_post_where, get_next_post_sort and get_previous_post_sort filters to
Replace the SQL query where and order by clauses string to sort adjacent posts by menu_order instead of post_date.

Resolves #67

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Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

Added filters callbacks for get_next_post_where, get_previous_post_where, get_next_post_sort and get_previous_post_sort to get proper results