Entity listing - add related posts counter

[mcolacino]

In the backend, add the # of related posts as custom column within entity posts listing

[mcolacino]

For the issue 1 we have to understand if have the new column sortable is a requirement or not: if it is, we have to choose how to implement it. We have (at least) two options

1. we can pre-calculate the total number of related posts and save the results on the db: it's the approach natively used for comments count sortable columns
2. we can calculate at runtime hacking the main wp query trough the posts clauses filter. This is why it's needed a custom join with the wl_relation_instances custom table.

[mcolacino]

Also for the issue 3 we have to consider if we need to pre-calculate the value or calculating it at runtime during the filtering: the first option allows better performances during the filtering but at the same time potentially introduces sync issues.

[ziodave]

For issue 1, this is the right way to do it (let's open a new issue for this specific implementation):

we can pre-calculate the total number of related posts and save the results on the db: it's the approach natively used for comments count sortable columns

[ziodave]

For issue 3, also it's better to "precalculate" it. Let's open a separate issue where we can discuss the implementation.