Add flag to database to track when service behind a layer no longer exists

[blewis]

We will keep a record for every layer we have known about in the past. After a new harvest, when it is discovered a layer no longer exists, we keep the record but flag it.

Handle this with a date column which is only populated with a date when a layer no longer exists?

These layers would (likely) no longer be pushed to Solr, though there could be value in an advanced search which finds cached layers which no longer have services behind them. Users who have loaded such a layer to their maps and have used the layer, may still have what they need in mapproxy. The fact the service layer behind the proxy no longer exists will be noted in the Layer page. Eventually users may be messaged that the layer they were using no longer has a server behind it.

[capooti]

@bglewis we need to have this in place before going to production.
So I am flagging it for the 0.3.5 milestone.

[blewis]

yes, thanks @capooti

[capooti]

moving to 0.4