Closes #2101 #2092

Adds a series of functions related to the commenting aspect of GitLab for the API wrapper. These functions are: `getComments`, `addComment`, `editComment`, `deleteComment`, `ensureComment` and `ensureCommentRemoval`. 

This reverts commit 7bc9a2db.

This reverts commit 29d00aa4.

Closes #2111

Closes #2073

Adds a wrapper to all datasources to provide a generic purl-based interface.

Adds range support for composer. Mostly leverages existing npm semver range support, but massages where necessary to support Composer differences.

Closes #2097

This PR adds the packagist datasource plus basic exact semver Composer support. Composer lockfile updating is not yet supported.

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