fix: upgrade multiple dependencies with Snyk

Snyk has created this PR to upgrade:
  - gatsby from 2.13.73 to 2.19.21.
    See this package in NPM: https://www.npmjs.com/package/gatsby
  - gatsby-image from 2.2.10 to 2.2.41.
    See this package in NPM: https://www.npmjs.com/package/gatsby-image
  - gatsby-plugin-catch-links from 2.1.9 to 2.1.25.
    See this package in NPM: https://www.npmjs.com/package/gatsby-plugin-catch-links
  - gatsby-plugin-manifest from 2.2.6 to 2.2.42.
    See this package in NPM: https://www.npmjs.com/package/gatsby-plugin-manifest
  - gatsby-plugin-no-sourcemaps from 2.1.1 to 2.1.2.
    See this package in NPM: https://www.npmjs.com/package/gatsby-plugin-no-sourcemaps
  - gatsby-plugin-offline from 2.2.7 to 2.2.10.
    See this package in NPM: https://www.npmjs.com/package/gatsby-plugin-offline
  - gatsby-plugin-react-helmet from 3.1.4 to 3.1.22.
    See this package in NPM: https://www.npmjs.com/package/gatsby-plugin-react-helmet
  - gatsby-plugin-sharp from 2.2.13 to 2.4.5.
    See this package in NPM: https://www.npmjs.com/package/gatsby-plugin-sharp
  - gatsby-plugin-styled-components from 3.1.11 to 3.1.19.
    See this package in NPM: https://www.npmjs.com/package/gatsby-plugin-styled-components
  - gatsby-remark-autolink-headers from 2.1.10 to 2.1.24.
    See this package in NPM: https://www.npmjs.com/package/gatsby-remark-autolink-headers
  - gatsby-remark-prismjs from 3.3.13 to 3.3.31.
    See this package in NPM: https://www.npmjs.com/package/gatsby-remark-prismjs
  - gatsby-source-filesystem from 2.1.21 to 2.1.48.
    See this package in NPM: https://www.npmjs.com/package/gatsby-source-filesystem
  - gatsby-transformer-remark from 2.6.22 to 2.6.53.
    See this package in NPM: https://www.npmjs.com/package/gatsby-transformer-remark

See this project in Snyk:
https://app.snyk.io/org/o330oei/project/17873807-1905-49d1-9bd2-207b42ee4164?utm_source=github&utm_medium=upgrade-pr