Fix: "Create private discussions with users that block it" permission not working for non-admin users

[rafaucau]

Changes proposed in this pull request:

• Fixed startPrivateDiscussionWithBlockers permission key
• It appears that there was a cannotBeDirectMessaged permission that was not declared anywhere and was always false (but for admin every permission is true). Because of this, for example, a moderator who has the permission to create private discussions with users who are blocking them could not create a private discussion with such a user despite having the permission.
• In addition, in this PR, I extended the types of the User model for type hints:
  

Reviewers should focus on:

To reproduce:

• Give Create private discussions with users that block it for non-admin user
• Login to that user account
• Go to user profile that blocks PD's
• On the current version of this extension, there is no button to create PD
• Check this PR, there will be now button to create PD if user has appropriate permissions

Screenshots

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes.