Add more context to invitation email

## Current design problem

The current email contain little context on what it's doing. Without looking at the sender address, it's impossible to understand it's from Authgear before inspecting the link.

Subject: `Collaborator invitation`
Content:
```
# Join [project] as a collaborator

You have been invited to join [project] as a collaborator. Please click the link below to view the invitation.

[View invitation]

If you want to reject the invitation please ignore this email.
```

## Change

Subject: `You are invited to collaborate on [project] in Authgear`
Content:
```
# Join [project] as a collaborator

You have been invited to join as a collaborator to manage the [project] account in Authgear. Please click the link below to view the invitation.

[View Invitation]

Authgear is a user identity and authentication software that helps you manage users and log them in securely.

If you want to reject the invitation please ignore this email.

```

Later when we make the name of user mandatory with a proper welcome screen, add the name of the person who invites to the subject too.

`[User] has invited you to collaborate on [project] in Authgear`