Make it a PWA

## Feature request

Gatsby apps are PWAs pretty much out of the box with the help of a couple of plugins. Is there a reason why this has not been implemented? You just need to add the gatsby-plugin-manifest and gatsby-plugin-offline plugins. This is how it looks in gatsby-config:

```
{
resolve: `gatsby-plugin-manifest`,
options: {
name: `GatsbyJS`,
short_name: `GatsbyJS`,
start_url: `/`,
background_color: `#f7f0eb`,
theme_color: `#a2466c`,
display: `standalone`,
},
},
'gatsby-plugin-offline',
```

I went ahead and implemented this in my own version and hit a snag because of a viewport error. Turns out the footer is exceeding the viewport width which triggers an error in Lighthouse. To fix this you need to set a max-width of 100vw on