Enable cycling header levels with ctrl+h.

closes #3019

* Remove preexisting `ctrl+h` keymap.
* Add logic to cycle the header level.
  * Loops around after h3.
  * Sets cursor position to end of heading
  * Set initial level to h2.
* Remove `ctrl+alt+${NUM}` keymaps (as they do not work
  internationally).
Updating markdown help modal with new shortcuts

issue #3019
Merge pull request #4145 from ErisDS/pr/3484

Enable cycling header levels with ctrl+h.