Problems storing truncated strings for Identity properties

[sduenas]

The current schema allows to insert values for the current Identity properties: source, email, name and username that due to their length are truncated to fit in the fields of the identities table. This is possible in DBMS like MySQL when STRICT_TRANS_TABLES or similar kind of parameters are not set.

There are two reasons to forbid this behaviour:

1. The values in the tables are different to those the user wanted to store
2. The UUID of the identity is invalid. Currently, the id of an identity is generated using the original values given by the user and not using the truncated values. This means the generated ID is wrong for a identity which its properties were truncated and we cannot recover the ID of one of these identities using the tuple (source, email, name, username).

Issue related #67