Global users can't recover their password from a subwiki

Steps to reproduce:

1. Create a subwiki with option "Both global and local users are available in the wiki"
2. In a private window, access to the subwiki and click on "Log-In"
   (http://localhost:8080/xwiki/wiki/subwiki/login/XWiki/XWikiLogin)
3. On the Log-in page, click on "Forgot your password?"
4. Put the email address of a Global user.
5. Even if this user is registered in the farm and has access to the wiki, the following message is displayed "The *** user does not exist"

It causes issues in a farm with several subwikis, and users sometimes forgetting their passwords. When they try to connect to a subwiki and want to recover their password, they don't manage to retrieve it.

Possible workaround: If the login is known as a global user one, redirect to the main wiki login page.
(http://localhost:8080/xwiki/bin/login/XWiki/XWikiLogin)

Relates to XWIKI-15681