Add input validation for length of entries into the Object filter fields in LDAP Configuration

When configuring LDAP user directory, the length of characters that can be entered for either User or Group Object filter is restricted by the Attribute_Value column of the CWD_Directory_Attribute table to 255 characters. However, the length of this input is not validated on the browser side. So when an object filter longer than 255 characters is entered into the field it fails against the database server causing an ugly SQL exception to be thrown at the user.

It's better to add validation for this entry on the client side for a few reasons:

• Although, standard databases would reject the input, Hsql for instance, did not enforce this column constraint and accepted the entry. This may cause failure in the future, if a user tries to migrate the same instance from Hsql to a production-worthy environment.
• The related bug report, regarding increasing the ammount of characters allowed for filters has been marked as wont fix (https://jira.atlassian.com/browse/JRA-24799), so having a user friendly message that warns the user about this limitation/restriction will be nice.