Issue #5: Updated models, created simple (debug, in progress) views and templates

[lilliputten]

Overall changes:

• Use standard content slot in templates.
• Created the list of user events render (template & view: vithout design).
• Updated python linter settings
• Added core constants module. Updated models (added model objects string representations). Updated project tools configuration.
• Used differrent python formatter (blue).
• Updated models. Customized user, registration and event admin models.
• Updated version: 0.0.4.