Tag to category and categories to tag converter links

[paolal]

In the Post Tags and Categories pages, links are made available to direct the users to the Categories and Tags Converter in Tools.

While it is normal that these links go to the Tools>Import page as long as the Categories and Tags Converter is not installed, once it is, the links should go one step further and bring the user directly inside the Categories and Tags Converter.

This was done deliberately at some point - see comment 5 on #13566. Checking for the plugin being installed and active would probably mean calling get_plugins() which might be a bit hefty. Doable, just a question of whether it's worth it?

I think it is worth trying to detect if the plugin is installed and linking straight to the page.

We can use is_plugin_active and/or is_plugin_active_for_network

If cat2tag converter is active, link directly to it

Patch checks if converter plugin is active.

If it is installed but inactive then it will link to the main importer page, which does some get_plugins stuff to detect if it is installed.