stac-check recommends Collection links have title field, but ones added by stac-server do not

[philvarner]

Running stac-api-validator gives the stac-check recommendation:

• [Collections] : GET https://earth-search.aws.element84.com/v1/collections/sentinel-2-l2a has these stac-check recommendations: Links in catalogs and collections should always have a 'title' field,

For example, the self link doesn't have a title. add these.