Rename writeToken to forkToken to make it clearer. This enables GitHub API to use one token for all reads, and another token just for writing to the forked repository.
This PR adds the capability to run Renovate in a new "fork mode". This new mode must be configured by the Renovate admin, and cannot be configured within repositories themselves (for now). Example use: `renovate --autodiscover --fork-mode`
In this mode:
* Renovate will fork the repository if necessary (first run only)
* If the fork already existed, Renovate will ensure that its base branch is up to date with the source repository's
* Branches will be created within the fork, PRs will be created in the source
Remove original getFile, getFileContent, and getFileJson and replace with getFile (which used to be getFileContent.. shortened now for convenience). Also remove unused getSubdirectories functions.
