Fixed some issues with subdir install of yourls

[phalox]

When installing yourls in a subfolder, there are a couple of issues with the this plugin

1st commit Made available the YOURLS_SITE config in js Issue:

• When Yourls is installed in a subfolder (e.g. your.ls/s/)
• The js code would generate the tag in the share box with url your.ls/srv/

Fix:

• Make available the full URL in javascript, prepend it

[phalox]

Fixes #39