Detect if custom content directory is used and if so, mention it won't fully work

[tsteur]

We should check if WP_CONTENT_DIR was customized. Need to see how to best check this. Maybe if(ABSPATH.'wp-content' != WP_CONTENT_DIR). To be seen.

If the directory is customized, the Matomo reporting UI won't fully work and the plugin should not do anything in that case besides showing a warning or an error basically. Or we could execute the plugin as usual and show only a warning on Matomo pages in WP that the Matomo reporting UI won't work. The summary reporting will still work very likely.