Fixes #2317

This reverts commit ba121836.

Adds the library `linkify-markdown` to actively linkify urls, issues and usernames so that they can then be linked to the renovatebot redirector for github.

Adds support for processing www-authenticate Header to obtain a token from the Docker servers. In particular, this enables Artifactory to work (without username/passwords for now). This is backwards-compatible to Docker Hub which issues this header with the current hard-coded URLs.

Removing the `yarn-path` option from `yarnrc` makes sure that renovate always uses its bundled version of `yarn`.

Closes #2310

Converts to HTML then replaces hrefs to github.com with renovatebot.com/gh/. Skips if non-github.com endpoint detected.

Closes #1804

Helps https://github.com/renovatebot/config-help/issues/67 and https://github.com/renovatebot/config-help/issues/70#issuecomment-407595535

baseBranch will be applied in PR title and in branch name only if there are more than one.

Closes #2306

Closes #2302

Adds description if onboarding PR is edited or conflicted. Removes config description if PR has been edited. Adds description of PR rate limiting.

Closes #1317, Closes #1684, Closes #1359

De-dupplicates text in the onboarding PR and makes it closer to what the theoretical commit message would be.

Closes #1315, Closes #2211

Closes #2304

Fixes #2283, Fixes #2119

Closes #2299

Closes #2291