Add ability to filter datasets by live status

[KarenJewell]

Stemming from OpenDataScotland/the_od_bods#26 The ability to filter datasets with working links (vs dead links/ archived items)

Question: do we need this? Should we even list datasets with a dead url anyway? Maybe they should be dropped in processing, so this feature becomes redundant.

We do have an existing function that checks url status (see analytics.ipynb) but because of the time it takes to process the whole listing, we restrict this to local authority published datasets only. So, function exists, probably not the best, but could be modified to be better, attribute does not exist yet as such.

Related to task OpenDataScotland/jkan#32

[JackGilmore]

@KarenJewell Proposing we close this as we don't list dead datasets on the site