Ability to reject an invitation to join an organization

[Houkime]

Describe the feature

Right now, any organization can invite any user and he/she will "join" that organization no matter if he/she wants it or not.

This can be used for spam.
(In reality there occured such a porn spam organization on notabug.org [can't provide a spam organization link since it was policed out])
It is also possible at try.gogs.io at the time of writing, which corresponds to 76b87b1 commit.
(Make an organization and add an arbitrary dude to it. No tricks needed.)

While strictly speaking not even a bug but rather a missing feature (and hence the feature request) this situation is annoying.

It would be nice for users to be able to confirm or deny invitations from possibly unwanted organizations.
It is especially needed if a user's organization list is publicly visible and unwanted organization can hang there for some time until the user or moderator steps in.

Describe the solution you'd like

When an organization wants someone to join, it should send this person a join invitation.
Then this user manually confirms it or denies it.
If the user confirms it, he joins organization.

Describe alternatives you've considered

One more possible solution is to reverse the situation.
So that it is only possible to add a user to an organization if the user him/herself sends a join request to an organization.