Adding validations for meta_* fields to match Database-constraints.

This fixes #5514. When using MySQL and inserting text into a string
field, the database will flunk if you commit over 255 characters.
This patch adds a few validations to make sure the various
meta_*-fields, using strings in the database, will contraint the lenght
with a maximum of 255 characters.

Fixes #5514
Fixes #5516