Rename close icon class to btn-close

Created by: alecpl

Imo:

1. close name is very generic, but adds a lot of styling and interferes with any use of such word for other purposes. Example: For buttons with icons I often use btn btn-secondary <icon-name>, If my icon is named close the button will get a lot of unwanted styles.
2. Close icon's use-case is generally a button, so why not btn btn-close?