Gatsby transformer remark doesn't work with assetPrefix

[xavivars]

Description

Gatsby transformer remark doesn't work properly when used together with assetPrefix. The plugin was not modified with #12128

Before assetPrefix was introduced, all relative links where always prefixed with pathPrefix. After assetPrefix was introduced, links are prefixed either with pathPrefix or assetPrefix, which is incorrect: assetPrefix should never be used to prefix links (see for example: https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-transformer-remark/src/extend-node-type.js#L206).

There are other problems also introduced by this: if the prefix is a fully-qualified-domain (meaning URLs are not relative), as that is not expected and pathprefix is always computed, urls become incorrect, due to some logic incorrectly replacing // with / in several places (https://example.com/foo becomes https:/example.com/foo).

https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-transformer-remark/src/extend-node-type.js#L324
https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-transformer-remark/src/extend-node-type.js#L63

Ideally, all URL management should be delegated to a proper URL builder class, to not to repeat the logic in different places.

Steps to reproduce

Create a simple project using markdown & assetPrefix, and create a relative link in markdown.

Expected result

Link should still be relative, with assetPrefix not included anywhere.

Actual result

assetPrefix becomes part of the URL

Environment

System:
OS: Windows 10
CPU: (12) x64 Intel(R) Xeon(R) CPU E5-1650 0 @ 3.20GHz
Binaries:
Node: 12.6.0 - C:\Users\xivarsribes\scoop\apps\nodejs\current\node.EXE
npm: 6.9.0 - C:\Users\xivarsribes\scoop\apps\nodejs\current\npm.CMD
Languages:
Python: 3.5.4 - C:\Users\xivarsribes\scoop\apps\python35\current\python.EXE
Browsers:
Edge: 44.17763.1.0
npmPackages:
gatsby: ~2.9.4 => 2.9.11
gatsby-image: ^2.1.4 => 2.2.4
gatsby-plugin-manifest: ^2.1.1 => 2.2.1
gatsby-plugin-offline: ^2.1.3 => 2.2.1
gatsby-plugin-react-helmet: ^3.0.12 => 3.1.0
gatsby-plugin-sharp: ^2.1.5 => 2.2.2
gatsby-source-filesystem: ^2.0.39 => 2.1.2
gatsby-transformer-sharp: ^2.1.21 => 2.2.1