Replace "register" with "create" in the context of adding a new nengajo to the contract

[alecrem]

Register/登録 → Create Nengajo/年賀状を作る

• [x] Navigation menu
• [ ] Within the register page
• [ ] Filename of the register page *

* I think it is OK to use "register" in code, but the page's filename goes on the page's URL.