Use a more secure approach to the invite flow

[mdtro]

Our invite flow does not behave in a way that users generally expect. Accepting invites with an already logged in Sentry user account is not expected by most of our users. Our original intent was to make it easy for a single Sentry account to be a part of multiple organizations.

Currently, if you're already logged in to Sentry and visit the unique invite link -- you're added to that organization regardless of whether your Sentry account uses the email address that the invite was sent to. This is the unexpected behavior users have raised concern over.

I'm going to revisit our invite flow and use a more secure/expected pattern.

Tasks

Beta Give feedback

1. Update invite flow to use a more secure pattern
2. Update email template to indicate that the link needs to be kept secure (e.g. do not share it)
3. Update our documentation to align with the new flow

[mdtro]

I believe the work done as a part of this issue will resolve #30065 as well.