Plots missing after tab refresh in jupyterlab 0.33

1. Install bokeh 0.12.16 or 0.13.0, jupyterlab_bokeh 0.6, and jupyterlab 0.33.
2. Run the following in a notebook cell and see the plot show up as expected.

from bokeh.io import push_notebook, show, output_notebook
from bokeh.layouts import row
from bokeh.plotting import figure
output_notebook()

opts = dict(plot_width=250, plot_height=250, min_border=0)

p1 = figure(**opts)
r1 = p1.circle([1,2,3], [4,5,6], size=20)

p2 = figure(**opts)
r2 = p2.circle([1,2,3], [4,5,6], size=20)

# get a handle to update the shown cell with
t = show(row(p1, p2), notebook_handle=True)

1. Refresh the browser tab.
2. When Lab reloads, the plots are missing.
3. Bokeh: ERROR: Unable to run BokehJS code because BokehJS library is missing shows in the JS console instead.

The plots did reappear properly after refresh in lab 0.32 with jupyterlab_bokeh 0.5.

[canavandl]

@parente a work-around is to use

from bokeh.plotting import output_notebook, figure, show

output_notebook(hide_banner=True)

which will load BokehJS without checking for the notebook content being reading, so that plots will render correctly on reloading.

[canavandl]

Thanks for the quality report @parente

Upon further investigation, it appears that the issue is the ```bokeh.io.output_notebook``` call embeds some JS in the notebook as part of a mime bundle. When the page is reloaded, this JS is evaled before the notebook is rendered, so the BokehJS loading is stopped here: https://github.com/bokeh/bokeh/blob/13baccbb2835f7d5831c22b5cf68214e89c02618/bokeh/core/_templates/autoload_js.js#L79