Fixed HoloViews axis linking across Template roots

[philippjfr]

In #826 I made some fixes to root handling but because I did some confusing things with a shared root a) plots were rendered too many times and b) you could not link axes across roots.

@timothydmorton @dharhas This one is worth testing on the LSST dashboard. I'm not sure you need to be able to link axes across multiple roots but if you wanted to you can do it again. It should also speed things up a little bit since it no longer renders plots multiple times.

[codecov]

Codecov Report

Merging #980 into master will decrease coverage by <.01%.
The diff coverage is 92.85%.

@@            Coverage Diff             @@
##           master     #980      +/-   ##
==========================================
- Coverage   85.74%   85.74%   -0.01%     
==========================================
  Files          98       98              
  Lines       11072    11077       +5     
==========================================
+ Hits         9494     9498       +4     
- Misses       1578     1579       +1

Impacted Files	Coverage Δ
panel/pane/holoviews.py	84.55% <100%> (ø)	⬆️
panel/tests/test_template.py	95.74% <100%> (ø)	⬆️
panel/template.py	70.9% <100%> (+0.53%)	⬆️
panel/io/server.py	40.62% <75%> (+0.62%)	⬆️

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