optimize matomo plugin loading

[diosmosis]

Matomo plugins are loaded as WordPress plugins, but unlike the standalone Matomo product, Matomo plugins may not need to be loaded on every request.

Avoiding loading plugins that don't need to be loaded within a specific context means every request will get a little faster. For example, for users that aren't logged in to the back office, only plugins that have tracking logic need to be loaded.

We should look into only loading what's necessary at the start of each request.