Deprecate old context properties and env vars

[rtsao]

Future versions of fusion-cli, fusion-plugin-react-router, fusion-react, and fusion-plugin-i18n won't use these context properties anymore, so their existence may be misleading or cause confusion.

• fusion-cli should provide the SSRBodyTemplate
• Simulation tests should assert against ctx.rendered and ctx.template instead of ctx.body, because the final HTML depends on the SSRBodyTemplate which comes from fusion-cli and depends on build-time things.

[codecov]

Codecov Report

Merging #299 into master will decrease coverage by 2.28%.
The diff coverage is 78.94%.

@@            Coverage Diff             @@
##           master     #299      +/-   ##
==========================================
- Coverage   91.91%   89.63%   -2.29%     
==========================================
  Files          18       18              
  Lines         408      434      +26     
  Branches       76       82       +6     
==========================================
+ Hits          375      389      +14     
- Misses         19       27       +8     
- Partials       14       18       +4

Impacted Files	Coverage Δ
src/server-app.js	100% <ø> (ø)	⬆️
src/base-app.js	88.97% <ø> (-0.09%)	⬇️
src/get-env.js	100% <100%> (ø)	⬆️
src/plugins/ssr.js	80.26% <67.74%> (-11.93%)	⬇️
src/plugins/server-context.js	93.54% <92%> (-6.46%)	⬇️

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