Fix "join" link on login page

Because there wasn’t a default value for $joinlink,  the “join” link
was linking to the homepage.
Make sure the lender's FirstName is a string (not NULL)

The column lenders.FirstName cannot be NULL.

Joining as lender was broken (in local setup) because the
$db::addLender tried inserting NULL in lenders.FirstName.
Merge pull request #42 from jonasdt/join

Fix "join" link and make registration