[WIP] Add support for dropdown to select the iTC testing group

[KrauseFx]

Note: I wasn't able to test this, as I didn't find a way to create a second group.

This PR adds a functionality to provide a white list of tester groups a user should be able to select from. This is useful in case you have multiple groups, for example Release Candidates and Nightly and want each user to choose which program they want to be part of.

The dropdown uses a white-list approach, as many dev teams probably have groups they want to use internally only (or maybe invite-only). This PR also makes sure that the user didn't manually add a new item to the request and try to add themselves by guessing names.
Also, the dropdown is automatically hidden if no white list is provided, or if there is only group provided. The group names are separated by the ; character.

Idea and lobbying by @steipete, :club_mate: works every single time :)

Missing pieces:

• I wasn't able to test this, as I didn't find a way to create a second group.
• Add documentation
• [optional] Add to app.json

[juliangrosshauser]

I just tested this and everything works as described. I was able to add myself to the group I selected.

[juliangrosshauser]

We deployed group selection to https://boarding-ios.pdfviewer.io if you want to check it out.

[juliangrosshauser]

Found a problem:

If I'm already registered in one group and try to add myself to another one I successfully get registered in the other group, but Boarding shows me the error message "Email address is already registered".

[juliangrosshauser]

Would also be super nice if you could specify some text to explain the different groups to users.

[KrauseFx]

Yeah, agreed, that sounds like two nice additions. Is that something you'd feel comfortable looking into? There is already an open PR that maybe we can merge together? #188

I'm happy to help out :)

[FinHorsley]

@KrauseFx any chance this could get merged? This feature would be incredibly useful for us

[KrauseFx]

I think this is ready to be merged @juliangrosshauser are you using this right now? Is it working as expected?

cc @joshdholtz

[FinHorsley]

@juliangrosshauser @KrauseFx also looks like https://boarding-ios.pdfviewer.io is down? seems to redirect to a page hosted by apple

[steipete]

We no longer use boarding, now that TestFlight basically does the same. It was really useful until then!

[FinHorsley]

@steipete How does TestFlight provide the same functionality, looks like i must have missed something
(or are you talking about the new API's?)

[joshdholtz]

@KrauseFx Are you still interested in getting this merged? 😉