Closes #2609

Adds terraform support for github sources with semver refs as well as fixed and ranged versions of Terraform registry modules.

Closes #1860

Adds a new datasource for querying terraform module registries.

Hashicorp’s semver is same as npm’s except for handling of “~> 1.2”. For Hashicorp it means ^1.2 whereas for Hashicorp is means ~1.2.

Closes #2603

Closes #2602

Closes #2596

Closes #2601

Closes #2600

Adds undocumented/experimental “master issue” feature.

Setting `config.masterIssue = true` will result in Renovate opening and maintaining an issue that contains a list of all PRs both pending and open, and allowing some control over them (e.g. bypass schedule, force retry, etc).

Setting `config.masterIssueApproval` in addition will mean that branches are not created automatically and instead await approval in that master issue.

Closes #2595