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 work 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