build css

looks like a recent PR didn't have a final render of the LESS
fix handling of no-checkpoint-defined

a few typos and missing handling of undefined checkpoint

regression introduced in #6205
minor fixes in dashboard tests

events and regular expressions weren't quite right, which could cause occasional test failures.
Merge pull request #6370 from minrk/handle-no-checkpoint

fix handling of no-checkpoint-defined
Merge pull request #6371 from minrk/dashboard-test-quickfix

minor fixes in dashboard tests
Merge pull request #6359 from minrk/build-css

build css
copy nbformat.v3 to v4
update v4 schema

use jsonschema refs, no need for jsonpointer
update v4 schema mime-type expression

fixes inappropriate JSON match
source is required in all cells
raise ValidationError in v4 validate

also only load the schema once and store it in a module global
first complete pass on v4

tests passing, notebooks validating
Use Draft4 JSON Schema for both v3 and v4

no longer need jsonpointer
no longer need separate v4.compose

put it back in nbbase, and work around circular import another way
mv stream.stream > stream.name in nbformat.v4

matches message spec
fix some validation bugs in v3

Python API had many inconsistencies with the actual spec
raise better ValidationError in validate

when oneOf would fail on a cell or output with an unhelpful error,
validate again against the target spec,
based on the output or cell type.
test and update conversion between v3 <-> v4
log, don’t raise on validation failure in v4.convert
update nbformat.current to v4