Explains activation code

[Indigo744]

As discussed in #1271

• Added comment to explains better how activation_code works
• Modify activate() function to extract the check activation code part, added as check_user_code_activation() function.
• As a bonus, fix a bug in which event post_activate_successful would not be triggered if an activation code was provided

[benedmunds]

This looks good to me.

@phalox can you take a look at this as well for any feedback you have

[phalox]

That's looking great, and exactly what I need :-) Let's merge and I'll update my code!

[benedmunds]

@Indigo744 is this good to merge? Anything else you want to change?

[Indigo744]

@benedmunds No I'm good 😸

[benedmunds]

Merged, thanks to both of you!