Use a more secure approach to the invite flow

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 are 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](https://www.linkedin.com/posts/biglan_in-general-great-product-sentry-sentryio-activity-7159255637476990976-G_PK?utm_source=share&utm_medium=member_desktop).

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

```[tasklist]
### Tasks
- [ ] When accepting an invite while logged in, only allow the invite to be accepted if the email it was sent to is a verified email on the account
- [ ] When registering an account via an invite, only allow the intended email to signup
- [ ] Only allow sending invites if the user has a verified email on their account
- [x] Update email template to indicate that the link needs to be kept secure (e.g. do not share it)
- [ ] Update our documentation to align with the new flow
```