Seeding ok with MySQL STRICT_TRANS_TABLE, fixes #157
Migrations for accounts and users tables

Now when clicking on INVOICE NOW, we can reach first step of a new
invoice creation even with MySQL 5.6 default STRICT_TRANS_TABLE.
Client import, create invoice with STRICT_TRANS_TABLE
nullable added to some fields
default integer value for payment_terms

Because sending an empty string to a integer column is not allowed with
MySQL in STRICT_TRANS_TABLE mode.
simpler short conditionnal

“Since PHP 5.3, it is possible to leave out the middle part of the
ternary operator. Expression expr1 ?: expr3 returns expr1 if expr1
evaluates to TRUE, and expr3 otherwise.”
http://php.net/manual/en/language.operators.comparison.php
3 constants taken from config
Hardcoded urls removed in mail templates
Merge pull request #158 from sdebacker/master

Compatibility with MySQL STRICT_TRANS_TABLE + removed some hardcoded urls, fixes #157