Field to specify whether a dataset can be downloaded using datalad

This issue proposes the addition of a new field to the `extraProperties` section of the CONP DATS.json file, named `download_options`. This field will indicate whether it is possible to download the dataset in question by the default datalad procedure as documented in the CONP interface page, and be used to determine which of the current "Direct download", "Download with Datalad" and "Third-party account required" buttons/links on the right-hand side of the dataset card are displayed and/or emphasised. Suggestions and other feedback welcome, this is very much an opening draft.

Suggested behaviour:

1) By default, when a DATS.json file does not contain an `extraProperties`=>`download_options` entry, the dataset card should be displayed as it is currently.
2) Datasets that require a third-party account for access should have that emphasised more strongly than the current design does, by removing non-functional "Direct download" and "Download with Datalad" buttons and adding a much more prominent "Third party account required" button, perhaps in a different colour from the existing green buttons in order to be immediately visible on the list of datasets. (Initial design suggestions: not red/amber that would suggest the dataset is unavailable rather than available by a different means; background colour should have a significantly different brightness level as well as hue to be similarly visual distinct to colour-blind users, which might also recommend dark text on brighter background?)
3) While initial design discussions have focused on this as a yes/no flag, to distinguish two categories of datasets which cannot be straightforwardly separated using existing fields, the `download_option` field could equally well support multiple values, to facilitate multiple alternative download button layouts. Is this an option we want to consider at this time?

Implementation:

Once the questions raised above are addressed to the point of implementing a prototype, this functionality calls for:

1) Annotating existing datasets that require a non-default `download_options` entry (such as OBI data.) A couple of these can be done quickly for initial test purposes to allow the other steps to proceed in parallel (Emmet).
2) Modifying the dataset display card to show redesigned buttons/links if a `download_options` value is read. (Laetitia)
3) Modifying the DATS editor to enable users to specify a `download_options` value (Alexandre) and come up with a clear and concise description of what it is for and when/why/how it should be filled in. (Patrick) Radio buttons or a pull-down list seem like workable options here.