use _safe_append for all output

catches js errors appending anything to output,
including javascript in an HTML output.

closes #4095
run test in parallel (FASTER)
added prettier printing
parallel is slowest of all, moving it to the front

suggested by @minrk
removing unnecessary print statements
add nbconvert config file when creating profiles
exclude log-level from ProfileCreate output
allow params in gh_api methods
add ability to check what PRs should be backported in backport_pr

just run:

    python tools/backport_pr.py 1.x

to see what PRs are marked for backport, but not yet applied.
remove unnecessary print statement
Merge pull request #4167 from ivanov/parallel-tests

--fast flag for test suite!
Merge pull request #4172 from minrk/backport-pr

add ability to check what PRs should be backported in backport_pr
Merge pull request #4171 from minrk/profile-create

add nbconvert config file when creating profiles
Merge pull request #4136 from minrk/catch-js

catch javascript errors in any output

not just javascript.

catches js errors appending anything to the output area, such as js in HTML (#4095).

closes #4095
remove special handling of speaker notes
clarify `reveal-prefix` alias name
update nbconvert docs about reveal.js
add proxying tornado server in Serve post-processor
use local reveal.js url prefix by default

CDN redirect is handled by the Serve post-processor if necessary
protect against unavailable tornado in post_processors.init