Logged in user can share their boards with other users (including private boards)

[CPowell23]

As a logged in user
When I visit my board show page
I see a link to share the board

When I click the link to share
I see a popup titled "Invite people to join this board"
The popup includes a text box where I can enter a name or email address of the person I want to share the board with

If I enter a name and click on "Invite"
An invitation is sent to the pinterest user with that name
Up for discussion: what do we do if there are multiple pinterest users with the same name?

If I enter an email and click on "Invite"
If the email belongs to a pinterest user, an invite is sent to them via pinterest
If the email does not belong to a pinterest user, an invite to join pinterest is sent to their email and the email includes a message summarizing the invitation to follow their friend's board