Fix collection inconsistencies on /content/add

[gmelodie]

Inspired by #168 this PR does the following:

1. fixes /content/add so it doesn't create CollectionRefs that have a null path field
2. changes the database to require a couple fields on collection_refs to be not null
3. makes coluuid and colpath query parameters on /content/add, not form values (consistent with other endpoints that use collections)
4. makes /collections/fs/list return an empty list instead of null when no entries are found

[en0ma]

makes coluuid and colpath query parameters on /content/add, not form values (consistent with other endpoints that use collections)

@gmelodie please make sure this change is done across shuttle, shuttle-proxy, barge, and any bash script in scripts dir

[en0ma]

@gmelodie I don't see shuttle-proxy and barge changes

[gmelodie]

@en0ma shuttle-proxy propagates query params so we don't need to update it, and barge doesn't seem to support collections (maybe add the support on a separate PR?)