[Feature] Enhancement of the Activity feature

[lcharette]

Issue with the current setup :

• Context is hardcoded (by the description)
• Activities are tied to a user, an cannot be tied to another object (ex.: a role)
• The description is not localized

Changes proposed :

• user_id is tied to the one doing the action
• Add a context_type, context_id for what the activity applies to
• Add a subject_type, subject_id for the action done on the context (nullable)
• Use the type for language translation
• Add metadata to pass additional context to the translation