Session management refactoring

locale/translations.go

locale/translations/fr_FR.json

model/session.go

model/token.go

server/cookie/cookie.go

server/core/context.go

server/core/handler.go

server/middleware/context_keys.go

server/middleware/session.go

server/middleware/token.go

server/middleware/user_session.go

server/routes.go

server/template/common.go

server/template/html/common/layout.html

server/ui/controller/controller.go

server/ui/controller/login.go

server/ui/controller/oauth2.go

server/ui/controller/session.go

server/ui/controller/settings.go

server/ui/controller/unread.go