Add dependency injection container

This patch introduces a dependency injection container for the PHP bug
tracker application. Container deals with the creation of all service
classe

Add dependency injection container

This patch introduces a dependency injection container for the PHP bug
tracker application. Container deals with the creation of all service
classes and additionally provides retrieving of configuration parameters.
Service classes are used everywhere in the app - from accessing database
to uploading files. Configuration parameters include infrastructure
configuration (database credentials...) and application level
configuration (directories locations...).

Container is compatible with the PSR-11 container interface defined
so it is simple to quickly understand its usage. Advanced features
such as autowiring are not included in this phase.

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