I18n internationalization in server, fix details and add es translation

[FedeG]

Changes

• Update documentation (Readme.md):
  • Add internationalization documentation to update translation
  • Add internationalization documentation to create new translation language
  • Add documentation of django >= 2.0 on urls.py examples
• Add i18n internationalization to server strings
  • Add verbose_name to all model fields
  • Add Meta class to models with single and plural name
  • Add labels to forms fields
  • Translate choices value on form
• Add string representations to all models
• Add translations to admin actions
• Add .gitignore file for django/python package
• Update en translations
• Create es translations
• Clean code and remove unused imports

[FedeG]

@skorokithakis could you check this pull request?
If you prefer that I divide it in partial pull request, tell me and I will rework it.

[skorokithakis]

Oh sorry, yes, I was intimidated by the sheer size of this. If you could split it up, that would be great.