Bugfix: make extension_url contain correct url in all cases

[ottok]

The old code populated _extension_url with contents like
https://example.org/var/www/sites/htdocs/wp-content/plugins/redux-framework/
-> ReduxCore/inc/extensions/customizer/

New implementation is does not mind if Redux is embedded in a theme or plugin,
supports having WordPress installed in any directory and to avoid future bugs:
the new code is clean and easy to read.

[kprovance]

The question becomes, what if one has their files in a place NOT wp-content? It's happened in the past. That also must be considered.

[ottok]

Why would the Redux code be outside of wp-content? It could be in a theme or plugin or must-use plugin, but in all cases in wp-content. Anyway, the old code was buggy, now it at least works for the WordPress plugin and theme use cases.

See also reduxframework/redux-framework#2754

[kprovance]

People sometime setup their wordpress to put content out of wp-content. It happens. Look it up. :)

[ottok]

Seriously, you need to have more precise argumentation to make you pull request review comments actionable, references to "look it up" does not help when I just argumented that plugins and themes are always under wp-content.

Please be precise what do you want me to do next before you merge these changes?

[kprovance]

I don't have to argue anything. However, I;m not in the mood to deal with more Redux users telling me how to run my project. @dovy said he would try it out in an environment that doesn't use wp-content. if it works, great. If not, overrules. It;s as simple as that. I'm sure he'll post back eventually. Cheers.

[ottok]

Hello!

I today updated my installation of Redux to 3.5.9 at our site (https://mariadb.org/) and it broken again due to the reasons this PR would have fixed. Any news if you are going to merge this?

[kprovance]

I've already explained several times the flaw in this approach. I'm not getting into it again.