JWT tween is too restrictive and block some read actions

including the personal feed in the homepage or the user profile pages.

Seen in the logs:
```
2018-01-31 20:08:10,891 DEBUG [c2corg_api.tweens.jwt_database_validation][waitress] JWT VALIDATION
2018-01-31 20:08:10,897 INFO [sqlalchemy.engine.base.Engine][waitress] ROLLBACK
2018-01-31 20:08:10,897 DEBUG [c2corg_api.tweens.rate_limiting][waitress] RATE LIMITING FOR METHOD OPTIONS
2018-01-31 20:08:10,898 DEBUG [c2corg_api][waitress] debug_authorization of url http://localhost:6545/personal-feed?pl=fr (view name '' against context ): Allowed (NO_PERMISSION_REQUIRED)
2018-01-31 20:08:10,907 DEBUG [c2corg_api.tweens.jwt_database_validation][waitress] JWT VALIDATION
2018-01-31 20:08:10,909 INFO [sqlalchemy.engine.base.Engine][waitress] BEGIN (implicit)
2018-01-31 20:08:10,910 INFO [sqlalchemy.engine.base.Engine][waitress] SELECT users.token.value AS users_token_value, users.token.expire AS users_token_expire, users.token.userid AS users_token_userid
FROM users.token
WHERE users.token.value = %(value_1)s AND users.token.expire > %(expire_1)s
LIMIT %(param_1)s
2018-01-31 20:08:10,910 INFO [sqlalchemy.engine.base.Engine][waitress] {'param_1': 1, 'expire_1': datetime.datetime(2018, 1, 31, 19, 8, 10, 907661), 'value_1': 'eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9.eyJleHAiOjE1MTg1MTUzNTEsInN1YiI6Mzc3LCJ1c2VybmFtZSI6ImFsZXgifQ.MY0Dg4MS3ySE-2YVVEwOVt55eHpIWWMc_Ohiv4rOJt4'}
2018-01-31 20:08:10,913 INFO [sqlalchemy.engine.base.Engine][waitress] SELECT users."user".blocked AS users_user_blocked
FROM users."user"
WHERE users."user".id = %(id_1)s
2018-01-31 20:08:10,913 INFO [sqlalchemy.engine.base.Engine][waitress] {'id_1': 377}
```