Create a list endpoint

[tcezard]

In ga4gh/refget#61 we discussed the addition of a list endpoint to provide the complete list of sequence collections contained in the database
The name of the endpoint should be /list and it would return a JSON array of level0 digests

[
    "lsuhfdow7dhsoshxhddjsiqpz",
    "2pqrt62xf0a72ffkbshw27sk",
...
]