Update the codebase with newer Elixir features

[IvanIvanoff]

There are quite a few places where the code can be updated. These include new features of Elixir/Erlang, removal of deprecated stuff or rewriting code in a way that aligns with the latest trends.

• Migrate away from the supervisor/2 and worker/2 functions commit from Supervisor.Spec
• Use handle_continue/2 in Blogit.Server.init/1, Blogit.Components.Posts.init/1 functions (possible other places, too) instead of sending message with . This avoids a possible race condition
• Clean the module that has use Application to not have so many functions
• Add .formatter.exs file and run the formatter

[jdmarshall]

deps.compile now fails with this code and I'm not even on the latest elixir.