Fix pagination on feed_issues endpoint

[brezocordero]

Changes proposed in this Pull Request:

Closes #936

Fix the pagination on the Feed Issues table by slicing the results based on per_page and paged parameters.

Screenshots:

Detailed test instructions:

1. Requires a site connected to Pinterest and a synched catalog with issues.
2. Go to wp-admin/Marketing/Pinterest
3. If your catalog has more than 25 issues, check the pagination works
4. If your catalog has less than 25 issues, add the parameters to the query and check the number of issues displayed is correct. (Ex: on a catalog with 18 issues page=wc-admin&path=%2Fpinterest%2Fcatalog&paged=2&per_page=10 should display 8 issues)

Additional details:

Changelog entry

Fix - Pagination on Feed Issues table.