Consider sanity limitations for all strings

[notshi]

For example, according to the standard:

transaction@ref An internal reference linking this transaction back to the publisher’s financial management system. This value must be of type xsd:string.

However, there are no character limitations which means the following is valid data. Source: Aidstream

And these are valid iati-identifier. Source: Registry

We found this because Postgres complains about text that is more than 8kb when creating indexes.

[andylolz]

This sounds like a proposed schema change. As such, it would be good to raise it on IATI Discuss, probably in "Modifications, Additions, Improvements": https://discuss.iatistandard.org/c/standard-management/modifications-additions-improvements