[emacs] assimilate slack and deps, clear up .gitignore, add secrets

hopefully the slack thing is temporary, as i don’t like using
proprietary platforms. however, if i must, i’d *much* prefer using
free software to access it, rather than executing proprietary JS.

note: since i’ve decided to commit etc/secrets.el, it shall not
contain any sensitive information itself, but rather how to obtain
them.  therefore, it must be byte-compiled; and the byte-compiled
etc/secrets.elc must *never* be committed.