allow to search by sample_name or ebi sample accessions

[antgonza]

This came up as part of creating a specific meta-analysis where the user only wanted to select a given set of samples from multiple studies, preparations and artifacts. The plan is to allow to search by only those fields and display the results using the regular study listing page, which includes a + sign to display available studies.

@ElDeveloper mentioned that in the current implementation we add a full artifact vs. a given sample; thus we will need to implement this part too.

cc: @ElDeveloper and @wasade

[antgonza]

Decided to add this "feature" to redbiom. The plan is to expose a new option in the to_dataframe method to add the accessions so this will allows to keep the same functionality we had before and add this flag to the redbiom loader so the new columns are added.