[4.x] Add ability to impersonate a user

[ryanmitchell]

This PR adds the ability to impersonate a user through a drop down menu on the user list:

There is an add-on for this, but it would be good to bring it into core.

Once impersonating it adds a little 'stop impersonating' icon to the bottom right, that @jackmcdade should really do some magic on to make it more 'rad'.

[jacksleight]

Hey Ryan, this is awesome!

A while back I submitted a PR to the addon that made the login quiet, that way the last login date doesn’t change when you start/stop impersonation (which IMO it shouldn’t). The implementation uses the same approach as Eloquent’s save quietly method.

Be great if this could do the same. What do you think? I haven’t tested this so apologies if that’s already covered.

[ryanmitchell]

Certainly. That should be there now.

[jasonvarga]

This is cool.

I know it's a draft but FYI when I click "back to my account", it just removes the impersonation status and I'm still logged in as who I was impersonating.

[duncanmcclean]

One thought: it might be good to have an option to disable this feature.

We have some sites where Statamic is used off the back of a Laravel app and we don't want admins impersonating users (they could change stuff they shouldn't be able to).

[ryanmitchell]

@jasonvarga that was me being dumb. Fixed now.
@duncanmcclean of course, anything for you.

This should be good apart from the styling of the button :)

[jackmcdade]

Thanks for this @ryanmitchell!

[ryanmitchell]

Thanks for making it look better!