Support for time series indices

Ponymail is basically time series data. It captures email data, including time sent. Time sent may make for a good key on a time series index.

https://www.elastic.co/guide/en/elasticsearch/guide/current/time-based.html

One of the downsides to elasticsearch is that your shard count can't change. There is an upper limit on total data stored in an index, based on the shard count and doc size. In order to allow ponymail to operate beyond those limits, it should store in an index keyed off of some factor. That factor is probably configurable - daily, monthly, yearly - or off. That index should be created automatically with settings standardized based on data volume.

No files, tests, or entry points are named. Start by locating Elasticsearch index creation and configuration, then trace how email timestamps and data volume are handled; done should include configurable daily, monthly, yearly, or disabled time-based indexing with standardized settings and automatic index creation.

Written by the indexing model from the issue text.