Shopping, consumable use and some fixes

[TemirkhanN]

• Lvl calculator fixed.
• All immutable data such as dungeons, monsters, droplist moved to data. It shall simplify adding new content with updates.
• Template engine(twig) provided(it is really hard to mix php and html without one). Every existing page is templated.
• Shopping system implemented (offers are kept in data/shop_stock.yaml folder). Grocery and armory for start.
• Item usage concept implemented. Works for cheese for now since it is the only consumable for now.
• API extended and isolated with a front controller. Adding new features is pretty easy now.
• A few more tests added.

New dependencies.
symfony/yaml - used for data parsing.
guzzlehttp/psr7 - used for api output.
twig/twig - used for pages rendering. Pushed towards segregating actions from representation(dungeon enter happens in ./src/UI/Scene/Dungeons.php while representation happens in ./src/UI/Template/dungeons.html.twig).

[crilleaz]

Will have to update index or add a wiki soon!