Minor fixes/improvements for 1.1.17

- Use role="presentation" for layout tables (Fix #8)
- Use url(https://) instead of url() for background color hack (FIx #5)
- Remove spurious line in editing after spacer block with different internal/external background option (Fix #3)
- Prevent "mobile rendering" on Yandex (workaround for a Yandex "bug") (Fix #1)
- Minor changes to remove unwanted spacing on iOS 10 (Related to #2)