Change how invite app works to work around Slack API limits

[StefanWallin]

Since Slack no longer allows access to the invitation API endpoint unless you are on an enterprise setup the easiest way forward is to message a workspace admin and have them manually invite people. To ease this we let our web app post a message into a Direct Message to an admin, or to a admin-shared channel.

This approach is in use by me at least. Take this if you wan't. Until Slack changes their mind it can at least be good workaround for others.

Note 1: I've changed out the library request to use cross-fetch instead since request is deprecated since 2019.
Note 2: The old approach might still be valid for enterprise setups, but so is this approach, but less automated.
Note 3: I've blindly edited all the configuration files, following existing patterns, since I do not know how to test all of them.

[StefanWallin]

Slack has come back to me, they are now telling me we could use the SCIM-API instead https://api.slack.com/scim#post-users. In this case we have to generate a password for the user or let the user enter their own password. This should be able to be used with a normal access-token that has admin-permissions. I've yet to try it out.