This PR ports across GitHub’s caching approach to the GitLab platform API.

This PR updates Renovate to detect config validation problems and (1) stop processing, and (2) either raise an Issue if already onboarded, or (2) update the onboarding PR to reflect the error if still onboarding.

Closes #1300 

Use a regex replace all instead of single replace, for cases where projects or branches have more than one forward slash.

Fixes #1293

* chore(deps): update dependency prettier to v1.9.0

* run prettier

Only block if it was closed unmerged.

Fixes #1205

* check for closed only for monorepos

Closes #1164 

Previously, Renovate's "Configure Renovate" onboarding branch would never get rebased after it was created. Now, it will be rebased every time the base branch is updated, unless the list of modified files is more than `renovate.json` alone.

Closes #1111 

GitLab’s API requires assignee *id* and not *username* when adding assignees to a merge request. Now, we allow Renovate users to still configure usernames and we will look up the ID and use it in the request instead.

Closes #1131

Remove original getFile, getFileContent, and getFileJson and replace with getFile (which used to be getFileContent.. shortened now for convenience). Also remove unused getSubdirectories functions.

Existing solution uses HTML bodies for PR descriptions, as that was the easiest way to get consistency between GitHub and GitLab. However, VSTS supports only markdown so we needed to refactor how this is done. Now, GitHub PR bodies uses only minimal HTML (for summary/details) while GitLab PR bodies are converted to HTML using GitHub flavoured markdown for maximum compatibility. VSTS will be able to strip out the minimal markdown.

Closes #1018

* chore(deps): update dependency prettier to v1.8.0

* reformat

Refactors file system of platforms and adds test to ensure GitHub and GitLab have same module.exports signatures.