Add ability to customize invitation text

* Invitation text field on multiple invite and Event#show pages
* Add helper method to show last invitation text in text field
* Disable invitation text field for non event owners
* Add tool tip to invitation text field
* Validate presence of invitation text

https://trello.com/c/muffB500
https://trello.com/c/gVg9SNuF
https://trello.com/c/ArPCCaSj