Include body, message and stack with every error if present

Fixes #2283, Fixes #2119

Pushes “getPrBody” logic into platform level to allow platforms to individually do things like text replacements and body length limiting.

Closes #2267

Closes #2141

Closes #2101 #2092

This PR adds the feature of commenting on a failed automerge. It's done by adding a conditional in `lib/workers/branch/automerge.js` which, in case of receiving `failure` or `error` from the `getBranchStatus` function, returns the "branch status error" value. Another modification is in `lib/workers/branch/index.js`, which is an adition to the failure response of the `tryBranchAutomerge` function. The added functionality is the ability to add a comment to the PR which had a failure automerging. In case of receiving the aforementioned "branch status error" value, to the comment is appended a note which emphasize the fact that there're multiple failed status checks.

Closes #1934 

Renames currentVersion to currentValue, newVersion to newValue, newVersionMajor to newMajor, and newVersionMinor to newMinor.

Closes #2032

This PR follows up and fixes #1968

The previous PR didn't behave correctly as it was expecting Github Enterprise to ALWAYS been configured and that the github.com token was available in the `GITHUB_COM_TOKEN` env variable.
But for non GHE project `GITHUB_COM_TOKEN` is not defined and github.com token is available at the `GITHUB_TOKEN` env variable instead.

This updated PR fix this issue and avoid further problems by prioritising `github.com` over GHE.
Now the code is NOOP if no `GITHUB_ENDPOINT` is configured.

If it's configured, instead, now the codes assumes that, by DEFAULT, a dependency is hosted on `github.com` so it removes `GITHUB_ENDPOINT` and use `GITHUB_COM_TOKEN` as Github token immediately.
They are restored only if needed, when a dependency is hosted on the provided GithubEnterprise.

This reverts commit f2fb2e6b.

Closes #1931

This PR splits the logic behind changelog into manager (npm) and source (github)
the manager provides the repo url + versions
the source consumes the manager info and generates changelog info

Closes #1911

Adds new option “pr-comment” for `automergeType` and a new option `automergeComment` to use in such a case.

Refactor changelog (commits) logic to separate sources, and remove the `changelog` dependency. Instead of a full copy/paste of commits, a link is now provided to the source repo.

Closes #381