Add elementId to html widgets in introduction vignette

[zkamvar]

Currently, the id attributes for the vis and map elements of the introduction look something like this: htmlwidget-080aed570e3f25a577b3, making the URL to link to these elements a bit unwieldy:

https://www.repidemicsconsortium.org/epiflows/articles/introduction.html#htmlwidget-080aed570e3f25a577b3

This PR adds explicit id attributes so that linking is a bit easier:

https://www.repidemicsconsortium.org/epiflows/articles/introduction.html#introduction-epiflows-vis

[cosi1]

@zkamvar Btw. did you downgrade some packages on purpose, or simply compiled the documentation on another machine? ;-)

[codecov]

Codecov Report

Merging #34 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #34   +/-   ##
=======================================
  Coverage   99.55%   99.55%           
=======================================
  Files          24       24           
  Lines         447      447           
=======================================
  Hits          445      445           
  Misses          2        2

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[zkamvar]

Ah! I compiled on a different machine, which caused the manual update to leaflet to revert 🤦‍.

That being said, as pointed out in the conversation over at leaflet (rstudio/leaflet#575), it really isn't much of an issue, but does cause warning bubbles to pop up for the maintainers (i.e. you and me).

[cosi1]

Sure :D