Issue with checking for is entity valid in follow tag context

Problem/Motivation

Inside the isValidEntity method of the FollowTagActivityContext and FollowTaxonomyActivityContext we check for every user which follow specific tag is he has access to view that entity. That is has a lot of performance issue on saving and update node, also on creating activity.

Proposed resolution

Instead of checking access to the node for all users, easier to check if the node has a field with relation to taxonomy. Also, add a alter hook to modify the list of fields that can be checked.