Enable PDFKit for certificate generation

* Both PDFKit and XHTML2PDF will work, but PDFKit is preferred. It can
  make prettier output.
* Documented new dependencies in readme, requirements
universal sharing and course completion/cert alert
Commit of Class2Go Fabric/Cuisine

Commit of Class2Go Fabric/Cuisine
universal share block with collapsing link if nothing special to share
removes blanks from quizdown multi-select

fixes #2416
Updates to Fabric/Cuisine PR

Updates to Fabric/Cuisine PR
Celery Config Fix

Fix to update chef settings that were left in
Merge pull request #2410 from Stanford-Online/jrbl-integrating_wkhtml

Enable PDFKit for certificate generation
Merge pull request #2415 from dglance/dglance-fabric2

Commit of Class2Go Fabric/Cuisine
travis IRC notifications: use_notice and skip_join = true
fixed conflict on master merge
Merge branch 'master' of https://github.com/Stanford-Online/class2go into giulio-share-results
travis IRC notification: fix yaml syntax, shorter templ
add default value to share_block_title var
fix whoopsie where I removed % sign from inline width attribute
fixed js error and added default share block content
Merge pull request #2423 from Stanford-Online/jbau-no-blank-multiselect

removes blanks from quizdown multi-select
remove unnecessary datetime import
move fabric/cuisine from required to suggested

Until this becomes the main way to do this let's call those two packages
suggested.
fixed typo in share block text