Add a `Number` field

Created by: gracewashere

https://trello.com/c/ElgpwKHW/30-when-i-visit-an-index-page-i-want-numbers-to-be-right-aligned-with-units-for-currency

Why?

• Many numeric values, like currency, need additional options to display correctly in the UI.
• We'd like to display numbers right-aligned in the UI, instead of left-aligned like other strings

Changes:

• Add Administrate::Field::Number
• Allow Number to take a prefix option
• Allow Number to take a decimals option
• Update generators to correctly assign integer and float fields a type of Number.
• Update changelog

Tags: #ruby