deploy_shinystan generates shiny code for private server deployment

[FBartos]

Added argument to deploy_shinystan function which makes it generate shiny code, which can be uploaded to a private server and run there. Also updated the vignette for deployment, with description how to upload the shinystan application to a private server.

note1: It's my first pull request ever. note2: I was having a trouble with "ppcheck_yrep" argument when specified, the posterior predictive checks don't seem to work. However, this should be unrelated to my changes.

[codecov-io]

Codecov Report

Merging #161 into master will decrease coverage by 4.53%. The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #161      +/-   ##
==========================================
- Coverage   97.09%   92.56%   -4.54%     
==========================================
  Files          10       10              
  Lines         896      847      -49     
==========================================
- Hits          870      784      -86     
- Misses         26       63      +37

Impacted Files	Coverage Δ
R/deploy_shinystan.R	90.42% <0%> (-3.26%)	:arrow_down:
R/misc.R	77.27% <0%> (-22.73%)	:arrow_down:
R/rhat_ess_monitor_fns.R	94.28% <0%> (-5.72%)	:arrow_down:
R/shinystan-objects.R	93.36% <0%> (-5.49%)	:arrow_down:
R/update_sso.R	93.75% <0%> (-3.62%)	:arrow_down:
R/launch_shinystan.R	35% <0%> (-3.1%)	:arrow_down:
R/generate_quantity.R	97.29% <0%> (-2.71%)	:arrow_down:
R/retrieve.R	97.33% <0%> (-1.37%)	:arrow_down:
R/sso-metadata.R	100% <0%> (ø)	:arrow_up:
... and 1 more

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