fix HTML capitalization in exporter classes
fix nbconvert filter validation
use register_filter() for registering user filters
use bootstrap `disabled` instead of `ui-state-disabled`

in menubar
make css highlight style configurable
Merge pull request #3512 from jakevdp/nbconvert_filter

fix nbconvert filter validation
Merge pull request #3507 from minrk/html

fix HTML capitalization in nbconvert exporter classes
use traitlet for highlight_class
Added relative import of RevealExporter to __init__.py inside exporters module.
Added another missing import inside transformer module.
Merge pull request #3520 from damianavila/import_reveal

Added relative import of RevealExporter to __init__.py inside exporters module
Merge pull request #3514 from minrk/ui-state-disabled

use bootstrap `disabled` instead of `ui-state-disabled`
Exclude frontend shim from docs build

Closes gh-3517
Allow specification of full highlight class in CSSHTMLHeader
Merge pull request #3523 from takluyver/docs-exclude-frontend

Exclude frontend shim from docs build

Closes #3517

The API doc writer has separate exclude patterns for modules and packages, so frontend needed to be listed in the modules exclusions.
Merge pull request #3516 from jakevdp/configurable_highlight

make css highlight style configurable