Invites: Improves translated string for invite header

[ebinnion]

Addresses part of #138 and related to #369

To test:

• Checkout update/invite-header-role-strings branch
• Invite a test user to join a WordPress.com site by going to Users > Invite New in wp-admin
• In the invitation email, make note of the invitation key
• Go to /accept-invite/$site/$invite_key
• Does the "invited you..." text seem appropriate for the role?
• Now, go back to the invite users UI in wp-admin
• Invite the same user, but with a different role this time
• Refresh the tab that has Calypso loaded
• Do the strings represent the new role?

cc @roccotripaldi for review and @rickybanister for wording review

[beaulebens]

invited you to be an author on:

[rickybanister]

They all seem great except this is weird:

invited you to author on

Let's go with

invited you to be an author on:

It doesn't match the others if we don't use 'author' as a verb, but I don't feel it's commonly used as a verb these days.

[roccotripaldi]

User invited you to administer: feels a bit strange. I'd prefer User invited you to manage:

[roccotripaldi]

Code looks great, and works as expected. I left one minor wording fix.

[rickybanister]

It appears this is already merged, but I don't think we should have switched to use the word 'manage'.

The role is 'admin' so I think using some derivation of that term is important.

has invited you to be an admin on: would be better than invited you to manage I think.