Support the paged argument in WP_Site_Query and WP_Network_Query

[birgire]

The WP_Site_Query and WP_Network_Query both support the offset and number arguments.

It would be handy to be able to use the paged argument, to make the pagination easier.

[birgire]

Support for 'paged' argument in WP_Site_Query and WP_Network_Query, with tests.

[birgire]

The patch adds the support for the paged argument in WP_Site_Query and WP_Network_Query.

It's similar to the approach in #38268, for the WP_Comment_Query, but I took a different approach in the tests. I tried to re-use the setup in:

• Tests_Multisite_Site_Query::wpSetUpBeforeClass() and
• Tests_Multisite_Network_Query::wpSetUpBeforeClass(),

using get_site_by() and get_network_by() to fix the test objects for the test queries. I will have to look better into that, if that's ok.

This patch also takes care of paged as 0, to avoid a negative start value in the LIMIT clause.

When it's 0 it's treated as the default value of 1.

[birgire]

Added the ticket number to the tests

[birgire]

Here we focus on the calculated offset in the LIMIT clause.

[desrosj]

Assigning to @spacedmonkey to shepherd.

After discussing this in today's ticket scrub, it was decided this is a good thing to add. It could help with Networks and Sites in the REST API, and adds parity to the other query classes in core. Will not make it into 4.9, but 5.0 is realistic.

[stevenkword]

There are a lot of tickets related to the LIMIT clauses. #42261 landed in feeds recently and @birgire has two other tickets open regarding similar issues. #41837, #41826