fix: add meta viewport tag

[nealrichardson]

Closes #535

[CLAassistant]

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[wch]

Am I correct that the <meta name="viewport"> is orthogonal to the Bootstrap dependency? If that's the case, then I think it makes sense to add a tags.meta() inside of the tags.head() here:

https://github.com/rstudio/py-shiny/blob/b874f0f0b33421d17a7dd6caac888e7b989fb445/shiny/ui/_page.py#L237

[nealrichardson]

I believe you're right but I'm not confident. I just looked for where it gets added in shiny in R and tried to do the same. Happy to move it elsewhere here if you think best (and presumably do the same over there too?)

[nealrichardson]

The internet confirms that this is an HTML5 thing and not a Bootstrap thing and it should probably always be there. But the place you suggested is still inside a call to page_bootstrap()--is that right?

[wch]

But the place you suggested is still inside a call to page_bootstrap()--is that right?

That is right. Now that I've thought about it a bit more, I think it does make sense to just put it in the Bootstrap dependency like you've done, since all Bootstrap users will want it. Also, that's also how we did it on the R side and that has gone smoothly.

@nealrichardson Can you sign the CLA Assistant thing? Then feel free to merge.

[wch]

@schloerke I think we need to do the same for shinyswatch?