[frame] prefetch codimd doc first, before downloading it

Otherwise, if doc wasn't visited yet, codimd returns some large HTML
page.
5 jobs for 333-dev-no-html-when-downloading-empty-note in 108 minutes (queued for 4 minutes and 10 seconds)
latest
Status Job ID Name Coverage
  Cabal
passed #12348
cabal

01:00:03

 
  Stack
passed #12349
stack

00:01:15

 
  Bench
manual #12350
allowed to fail manual
bench
 
  Test
passed #12360
test

00:46:40

failed #12351
test

00:11:13