Customised placeholder for customer metadata attribute

[whomingbird]

Currently the placeholder text of the customer metadata attribute is defined through SampleAttributeType. Each type has the same text.

e.g. int_type = SampleAttributeType.find_or_initialize_by(title: 'Integer') int_type.update_attributes(base_type: Seek::Samples::BaseType::INTEGER, placeholder: '1')

It would be nice to have the different placeholder texts for the different customer metadata attributes.

E.g. the min age is "e.g. 5", the max age is "e.g. 80"

[floradanna]

Relevant for sample type attributes as well. Try to keep customised metadata and sample type consistent. Consider how to include this in the GUI to make attribute in sample type.