Updated code for new purchasable implementation

* Using the new repository for purchasable instead of all single ones
* Adapted store and templates for this change

Fixes:

* Class ProductHasOnlyOneCategoryEventListener fixed. Doctrine was crashing because of bad practice of collection setters.
  More information in doctrine/orm#5601
* Fixtures restarted. No one resultant fixtures cannot be applied when there is some data.

Code:

* Code has changed in order to establish a new relation between the project and elcodi updates.
* All admin pages, twig templates and forms have been created
* Store have changed in order to work with new implementation
* Some names were explicitely refering to product instead of purchasable. This has changed.
* Using purchasable_name and valid_image twig extensions in store template

Tests:

* Some tests have been redone. All of them were testing behavior, and because some urls have been changed, for example,
these scenarios have been changed.
* Added some entries in Visithor and fixed old ones

Translations:

* Updated the translations guide [EN]
* Fixed some keys for purchasable_pack
* Added serbian in Languages and fixed Galician

.translation.yml

@@ -11,5 +11,6 @@ languages:
     - gl
     - eo
     - nl
+    - sr
 paths:
     - src/Elcodi/*/*/Resources/translations