Draft: Make all table headers sticky

Follow up from !1593 (merged), this moves the "sticky table heading" style up to all tables, not just wide table.

Homepage:

• Before: https://docs.gitlab.com/ee/README.html
• After: http://eread-make-all-table-headers-sticky.178.62.207.141.nip.io/ee/README.html

Large permissions table:

• Before: https://docs.gitlab.com/ee/user/permissions.html#project-members-permissions
• After: http://eread-make-all-table-headers-sticky.178.62.207.141.nip.io/ee/user/permissions.html#project-members-permissions