Add method to build district + buy tile in one click

**Issue by [chaorace](https://github.com/chaorace)**
_Wednesday Mar 01, 2017 at 00:59 GMT_
_Originally opened as https://github.com/CQUI-Org/cqui/issues/360_

----

Currently, you need to buy a tile, THEN choose where a district goes. This could be streamlined to just choosing to build a district on a purchasable tile, which we could interpret as "buy then build on this"

Start by tracing the existing tile-purchase and district-placement UI entry points, since the issue names no files or tests. Compare the current two-step flow and determine how a district selected on a purchasable tile should trigger the purchase first. Done means a player can choose a district on a purchasable tile and complete both actions through one interaction.

Written by the indexing model from the issue text.