Ensure ReactiveHTML correctly interfaces with managed layouts

[philippjfr]

• Do not compute managed layout unless your parent is a managed layout
• Do not set managed layout position unless your parent is a managed layout
• Error if any tag in the template is unclosed

[codecov]

Codecov Report

Merging #2402 (38f48b9) into master (3eb9a63) will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #2402      +/-   ##
==========================================
- Coverage   83.45%   83.45%   -0.01%     
==========================================
  Files         185      185              
  Lines       22678    22680       +2     
==========================================
+ Hits        18927    18928       +1     
- Misses       3751     3752       +1     

Impacted Files	Coverage Δ
panel/reactive.py	74.96% <50.00%> (-0.07%)	⬇️

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