Add placeholder text on query boxes

[kjolley]

It should be possible to customise the greyed-out "Enter Value..." indication for each of the forms in order to provide the user with an example of what he is supposed to enter in the search box.

This could be particularly useful when searching for certain nomenclature with a nickname such as Sublineage for Klebsiella to indicate to the user that they should search for "SL15" and not just "15".

The value will need to dynamically update depending on the field selected and should be an option on all form elements (particularly provenance and secondary metadata fields).

[Suggested by Martin RETHORET-PASTY, Pasteur]

[kjolley]

Implemented in v1.48.0.