Use a `sortOrder` attribute in ReaderPost model instead of overriding the timestamp

[mzorz]

Expected behavior

ReaderPost model should have a new attribute that represents the sort order weight for a ReaderPost instance, and all other classes using it should reflect this accordingly

Actual behavior

Right now timestamp, being used as a sorting criteria, even when sometimes a value other than an actual timestamp is assigned to such timestamp attribute, which is misleading.

Steps to reproduce the behavior

Check conversation here https://github.com/wordpress-mobile/WordPress-Android/pull/4096/files/0cc5bd209d8ebf0deeff93d716ca194d2eb0df4f#r64293763

[nbradbury]

Closed via #4160