Invites: Fixes error when attempting to accept follower invites

[ebinnion]

While testing a follower invite, I noticed that the invitation never fully loaded because of a JS error:

Uncaught TypeError: this.getSiteName is not a function

This PR fixes that by adding the method back.

cc @lezama

To test:

• Checkout fix/invites-get-site-name-error branch
• Create a follower invite on a public WordPress.com site by going to $site/wp-admin/users.php?page=wpcom-invite-users
• After the page reloads, copy the link address for resending the invitation.
• Make note of the $invitation_key
• In the invitation email that you get, make note of the $key and the $activation parameters in the activation link
• Go to /accept-invite/$site/$invitation_key/$activate/$key
• Are there JS errors? No? Good.

[enejb]

Where is this new function being used?
also why not just
return this.props.site || this.props.site.title;

[ebinnion]

That function was an already existing function, which was then mistakenly removed. 😞

So, you'll see that the function is getting called in a few places throughout the same file.

We return '' if a site title doesn't exist because we are using the title in translations and don't want an object.