Invitation email is confusingly worded for existing users

[jml]

When someone adds you to an instance, you aren't really "invited"—there's nothing to accept or reject. This leads to confusion.

We should send one kind of email when inviting people who aren't existing users, and another kind of email when granting access to someone is a user.

Also, while fixing this, it would be good to ensure that the invite email says "organization" when it should say "instance".

Probably don't fix this until the PRs starting #678 are merged, as they refactor some of the invite stuff.

Part of #618.

[jml]

Since we have to make these changes on sendgrid, I thought it would be best if someone could review my prose here:

Current wording

You've been invited to join the organization ":org_id" on Weave Cloud.

Click this link to login to Weave Cloud:

Log in to Weave Cloud

This single-use login link is valid for 3 days. You can generate a new one at any time by visiting Weave Cloud

You need to run probes on your Docker hosts to send information to Weave Cloud. Instructions for that are available once you log in.

Thanks, the Weaveworks team

<%body%>

Proposed wording

First time

:user_name has invited you to access the ":org_name" monitoring instance on Weave Cloud.

Click this link to login to Weave Cloud:

Log in to Weave Cloud

This single-use login link is valid for 3 days. You can generate a new one at any time by visiting Weave Cloud

You need to run probes on your Docker hosts to send information to Weave Cloud. Instructions for that are available once you log in.

Thanks, the Weaveworks team

<%body%>

Differences

• org_name not org_id
• include user name
• "monitoring instance" not "organization"

Already have an account

:user_name has granted you access to the ":org_name" monitoring instance on Weave Cloud.

View :org_name

Thanks, the Weaveworks team

<%body%>

Differences:

• No mention of "logging in", assume they have that down
• No mention of Docker probes, because the instance already exists
• Mention user_name
• Use org_name, not org_id
• Link directly to the specific organization

[paulbellamy]

We don't always have a user_name. There is always a user email, but only a username if they've logged in via github (or google).

[jml]

Thanks. I'll amend that.