Refactored server handling

[philippjfr]

This PR refactors the server handling such that all existing methods that start a bokeh server simply call the new Viewable.get_server method and then publish the server session in different ways. This also means that we now take full control over embedding servers inline in the notebook, which makes it possible to automatically clean up server sessions as cells are deleted or re-executed.

Depends on new versions of both the JLab extension and pyviz_comms: holoviz/pyviz_comms#28

[codecov-io]

Codecov Report

Merging #276 into master will decrease coverage by 0.52%.
The diff coverage is 31.2%.

@@            Coverage Diff             @@
##           master     #276      +/-   ##
==========================================
- Coverage   88.25%   87.73%   -0.53%     
==========================================
  Files          58       59       +1     
  Lines        5712     5754      +42     
==========================================
+ Hits         5041     5048       +7     
- Misses        671      706      +35

Impacted Files	Coverage Δ
panel/models/plots.py	100% <100%> (ø)	⬆️
panel/pane/holoviews.py	82.35% <100%> (ø)	⬆️
panel/__init__.py	100% <100%> (+45.83%)	⬆️
panel/layout.py	95.2% <100%> (ø)	⬆️
panel/pane/base.py	85.88% <100%> (ø)	⬆️
panel/viewable.py	65.04% <19.23%> (-3.29%)	⬇️
panel/util.py	67.33% <23.52%> (-3.23%)	⬇️
panel/io.py	38% <38%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update f327c1b...c04b787. Read the comment docs.

[philippjfr]

Fixes #231