Wrong share link generated when using different base URL

[galagithub]

Steps to reproduce:

1. host pigallery2 under a different base url (other than "/") and configure it in the settings
2. generate a "share" link, it won't contain the base url in the link and thus it cannot be reached.

For example:

• full address "http://foo.moo/bird"
  pigallery2 generated share link will be "http://foo.moo/share/xyz1234" which will not work
• if you manually change it to "http://foo.moo/bird/share/xyz1234" then it works

[galagithub]

Pull Request already made for this:
#1170