Implementing deactivate user

[LikitSaeLee]

reference: #87

IMO, The rules should be evaluated by specificity.

1. User specific rule.
2. Group rule.
3. Percentage rule

Example:

$rollout.activate_group(:chat, :all)
$rollout.deactivate_user(:chat, user)
$rollout.active?(:chat, user) # false, because user specific rule takes highest precedence

$rollout.activate_group(:chat, :group_including_user)
$rollout.deactivate_user(:chat, user)
$rollout.activate_group(:chat, :another_group_including_user)
$rollout.active?(:chat, user) # false, because user specific rule takes highest precedence

$rollout.deactivate_group(:chat, :group_including_user)
$rollout.activate_user(:chat, user)
$rollout.active?(:chat, user) # true, because user specific rule takes highest precedence

Changes:

1. Denote deactivated user id in feature#users by appending x in user_id.
2. A single user id can only be added (A user cannot be both active and deactivated)
3. Proper implementing the rules above.
4. Add Rollout#remove_user so that the user_id/user specific rule can be removed ( for reducing memory usage on redis ).

[cohendvir]

hey @LikitSaeLee, is this pr going somewhere? :)
we just had a bug in a feature we deployed for 10% of our users, and we had to roll it back completely because we couldn't turn it off for that one user..
This pr could have been really helpful right now!

[raghuvarmabh]

any update on this. I am also in the same boat.

[ErebusBat]

As @reneklacan stated in #87 (comment):

I'm kind of afraid this would introduce even more unexpected behaviour if it's not well thought out.

This PR has clearly defined rules; however this PR does change the well defined and understood behavior of existing functions. For that reason alone I would be hesitant to give this PR a 👍.

we just had a bug in a feature we deployed for 10% of our users, and we had to roll it back completely because we couldn't turn it off for that one user.

The way we have addressed this in the past has been to create a specific disabling flag rollout. While this can seem messy at some points it is actually quite nice because after all the bugs are worked out the disabling rollout can be 100% deleted. If you keep the original feature rollout then you can check the activation percentage and know, without a doubt, that it is enabled for everyone.