Add favicon/manifest

[missmatsuko]

Generates favicon/web app icon from single source SVG and adds web app manifest.
Closes #191

What kind of change does this PR introduce?

Feature

Does this PR introduce a breaking change?

What needs to be documented once your changes are merged?

[ZoltanVeres]

@missmatsuko The build for your first push was 6 mins and the second push took 10 mins to deploy.
Honestly not sure if this is Netlify related or the plugin has its own issues. May i ask how this affected the build time on your own repo?

[missmatsuko]

I was looking at the build scripts and I think build is going to delete all the files in public regardless if they were committed or not? This might be why committing the generated files aren't having an affect on build time.

"clean": "rimraf .cache public",
"build": "npm run clean && gatsby build",

[missmatsuko]

I took a look at the gatsby-plugin-manifest, and I think it doesn't check if generated files already exist before running. Make an issue on Gatsby to see if they might want to add this feature:
gatsbyjs/gatsby#12506

[ZoltanVeres]

@missmatsuko First of all, thank for taking the time to figure out this issue. We appreciate each PR, I think our problem in the starter is that we should fix the caching in the build process, and an ideal tool would be to use gatsby-plugin-netlify-cache, but i would wait until this issue is cleared. axe312ger/gatsby-plugin-netlify-cache#15