Analytics - Deployments per day Monthly Average

**Is your feature request related to a problem? Please describe.**
As teams begin to use DeployBoard for multiple months, we would like to have an easy display of the monthly trend. Currently the granularity is only by day, and going back 6-12 months is going to be a very dense graph.

**Describe the solution you'd like**
Create a graph on the /analytics page that finds the deployments in a month, adds them together and divides by the total days in that month. The API route should return something like {'month':'count'} that the UI will simply bar or line graph.

**Describe alternatives you've considered**
None

**Additional context**
Eventually we want to give the user a toggle to swap between line and bar graphs, but we'll have to standardize to one of them for now.

Start by locating the existing /analytics API route and the analytics page that currently displays daily deployment data. Trace how deployments are grouped and rendered, then add the monthly result in the requested month-to-average shape; done means the page displays the monthly averages over longer periods without losing the existing daily view.

Written by the indexing model from the issue text.