Keymaps refactor

this PR consists of two parts:
* move keymaps to separate files, kill duplicating code
  * 1cac49a messages: refactor message history keymaps and bindings to msg-history.rkt
  * 23faf3b chat: groups: refactor editor keymap to separate module, also create utils.rkt with helpers

* add killing word to the left/right of cursor and text selection with shift
  * 0a88cfb message editor: add forward/backward char selection
  * 246f842 message editor: add backward and forward kill word