Deploy and test the new rendering code added in {T171838}.
[x] configure `$wgVirtualRestConfig['electron']` (https://gerrit.wikimedia.org/r/#/c/377928/)
[x] set the secret key in the private repo
[] test functionality on beta
[] backport rendering patch https://gerrit.wikimedia.org/r/#/c/361453/ (cherry-pick: https://gerrit.wikimedia.org/r/#/c/377936/ )
[] find some way to prevent clever users who discover the non-public test URL from using it (https://gerrit.wikimedia.org/r/#/c/377929/)
[] test with some large books (e.g. from T142226#2537844)
* check MediaWiki memory use
* check Electron memory use (run `top` on scb1001-1004)
* check service latency
* make sure the test did not cause problems for Electron / restart if needed
----
Rendering workflow is as follows:
* read book definition from session
* fetch HTML for pages from RESTBase via MultiHttpClient; fetch metadata (sections, contributors etc) from MediaWiki API via internal pseduo-request
* compose all those into a single HTML page with Remex
* store book data in WAN cache with a short TTL
* fetch PDF from Electron
** Electron will send a request to MediaWiki, MW will load the book from cache and render it.
* output the PDF response with the appropriate headers (the current code does not try to stream the response; that should probably be added)
----
Restarting Electron:
```
for i in 1 2 3 4; do echo $i; ssh scb100$i.eqiad.wmnet "sudo service pdfrender restart"; done
```
There is an Icinga alert when it goes down. ([[https://icinga.wikimedia.org/cgi-bin/icinga/extinfo.cgi?type=2&host=scb1001&service=pdfrender|scb1001]], [[https://icinga.wikimedia.org/cgi-bin/icinga/extinfo.cgi?type=2&host=scb1002&service=pdfrender|scb1002]], [[https://icinga.wikimedia.org/cgi-bin/icinga/extinfo.cgi?type=2&host=scb1003&service=pdfrender|scb1003]], [[https://icinga.wikimedia.org/cgi-bin/icinga/extinfo.cgi?type=2&host=scb1004&service=pdfrender|scb1004]])