Custom turn blockers

**Issue by [plucko00](https://github.com/plucko00)**
_Wednesday Mar 08, 2017 at 19:12 GMT_
_Originally opened as https://github.com/CQUI-Org/cqui/issues/381_

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So often I mean to change my policy cards before the end of the turn but forget and then have to wait till the next civic or pay gold. Could there be an option to turn on a block next turn if you have the option to change them but haven't yet. And obviously a way to bypass it if you don't want to.

Start by locating the Lua code that handles turn completion and policy-card changes, then trace how existing turn-ending prompts or blockers are implemented. Define the opt-in blocker and its bypass behavior, and verify that a player can continue normally when the option is disabled or bypassed.

Written by the indexing model from the issue text.