add a "preview" option to the "edit charter" page

[paulwouters]

Description

add a "preview" option to the "edit charter" page so we can verify markdown/txt conversion to html before submitting a charter. This avoids silly revisions to be created by incompetent ADs like me :)

Code of Conduct

• [X] I agree to follow the IETF's Code of Conduct

[rjsparks]

This should be provided wherever we accept markdown when someone is logged in.