User App enhancements

[ZeyadKhattab]

I will write here all the improvements based on PR #44

• H: too late should be in base class before any verification of parameters. Same for similar commands. Code duplicated as well.
• M: function belongs to Arango module. It should be there.
• M: could add a method prestart to Service and override it in this class to ensure uniformity across apps.
• Add uriParams validation in CommandWithVerification
• Builder Pattern for User model
• PostgresConnection is achimera of static and instance methods. Not fully a static class and not fully a Singleton class either.
• Proper testing of PostgresConnection
• Main Method should not throw an exception
• graceful shutown if env variable not loaded properly
• Use Attribute class to get the parameter from the body
• use try-with-resources or close this AutoCloseable ](User App #44 (comment))
• Transaction-like behavior when using 2 different DBs
• Users' deletion discussion

[ZeyadKhattab]

Fetching the number of followers in GET Request #54