Slack invite api

Send a request to slack api (as admin). The api will be return json like this:

    {
      "ok": false,
      "error": "already_invited"
    }

This method has the URL https://slack.com/api/users.admin.invite and follows the Slack Web API calling conventions.

Argument	Example	Required	Description
token	xxxx-xxxxxxxxx-xxxx	Required	Authentication token (Requires scope: `'client'`)
email	john.doe@email.com	Required	Email address of the new user
channels	C1234567890,G12345678	Optional	Comma-separated list of IDs (not names!) for channels, which the new user will auto-join. Both channel IDs for public channels and group IDs for private chanels work.
first_name	John	Optional	Prefilled input for the "First name" field on the "new user registration" page.
last_name	Doe	Optional	Prefilled input for the "Last name" field on the "new user registration" page.
resend	true	Optional	Resend the invitation email if the user has already been invited and the email was sent some time ago.
restricted	true	Optional	Invite a guest that can use multiple channels
ultra_restricted	true	Optional	Invite a guest that can use one channel only

{
"ok": true
"error": "already_invited"
}

ok : this will be return true or false when ok, slack will send an email, if false, read line bellow
error: if ok == false, error will be return error code

Error	Description
`already_invited`	User has already received an email invitation
`already_in_team`	User is already part of the team
`channel_not_found`	Provided channel ID does not match a real channel
`sent_recently`	When using resend=true, the email has been sent recently already
`user_disabled`	User account has been deactivated
`missing_scope`	Using an access_token not authorized for `'client'` scope
`invalid_email`	Invalid email address (e.g. "qwe"). Note that Slack does not recognize some email addresses even though they are technically valid. This is a known issue.
`not_allowed`	When SSO is enabeld this method can not be used to invite new users except guests. The SCIM API needs to be used instead to invite new users. For inviting guests the `restricted` or `ultra_restricted` property needs to be provided
tbd