feat: skipInstalls (#2390)

Adds new admin option “skipInstalls” that is applicable for npm-only for now (including lerna-npm). If set to false, Renovate will perform a full install of modules rather than `—package-lock-only`. This is necessary in some cases to work around bugs in npm.

Self-hosted bot users can set this option themselves on the bot’s config, but app users will require it to be enabled per-repository by the app admin.