People: Properly translate role based strings

[v18]

We display role based strings in a few places in the accept invite flow, and we'll need to make sure that these are able to be translated properly.

Example: "Sign up to become an editor on Busy Mom Confessional?"

We need to properly build that string so that it returns returns a or an for English. Ex. - an editor or a contributor.

My guess is that we should switch on the role, and then for the ~5 roles that we would expect, we should return a translated string specific for that role.

ex.

let text = '';
switch ( role ) {
    case 'administrator':
        text = this.translate( 'Sign up to become an Admin on...' );
        break;
    case 'editor':
        text = this.translate( 'Sign up to become an Editor');
        break;
    case 'author':
        text = this.translate( 'Sign up to become an Author');
        break;
    case 'contributor':
        text = this.translate( 'Sign up to become a Contributor');
        break;
    case 'subscriber':
        text = this.translate( 'Sign up to become an Subscriber');
        break;
    default:
        text = role;
}

return text;

cc @yoavf for advice on if the switch above is a reasonable approach.

Another example of strings that we'll need to properly translate is the invite header. Based on roles, we'll want to create strings such as:

let text = '';
switch ( role ) {
    case 'administrator':
        text = this.translate( '%(user)s invited you to administer %(blogname)s' );
        break;
    case 'editor':
        text = this.translate( '%(user)s invited you to edit %(blogname)s' );
        break;
    case 'author':
        text = this.translate( '%(user)s invited you to author on %(blogname)s' );
        break;
    case 'contributor':
        text = this.translate( '%(user)s invited you to contribute to %(blogname)s' );
        break;
    case 'subscriber':
        text = this.translate( '%(user)s invited you to subscribe to %(blogname)s' );
        break;
    case 'follower':
        text = this.translate( '%(user)s invited you to follow %(blogname)s' );
        break;
    default:
        text = role;
}

return text;

To be honest, some of those string feel weird. Perhaps we can reword them? Also, we may be able to draw some inspiration for wording from the invitation emails.

cc @rickybanister for thoughts on wording