Do not show "Start new group" to all new users

[sellth]

What is your feature request?

Currently when a non-HPC user logs into HPC Access the "Start new group" screen is shown. As most users are in fact not PIs and this makes for a confusing first experience and will lead to spam. The initial screen should probably give a choice:

"You currently do not have a cluster account. What do you want to do?"

1. Create new group (PI)
2. Join an existing group

Option 2 can just show a static message telling the user to have a group PI/Delegate apply for a new user account. In the future we could also implement group join requests so the users can self-service.