[plans] Fix invite team member URL

[svenefftinge]

The invite URL doesn't work atm.

Repro:

• purchase team plan
• copy invite URL
• use URL in another browser with a different user

currently, I end up on the plans page with no information about the team plan.

Designs

For the confirmation modal before accepting the invitation here're the design specs.

Invitation (Prompt)	Invitation (Error)

See also design specs.