Enable basic-code-intel by default

Created by: chrismwendt

This introduces Default settings, which is a new level of settings at the lowest level of precedence in the settings cascade (below global settings):

• Default settings (new)
• Global settings (A.K.A site settings in the Go code)
• Organization settings
• User settings
• Client settings

I hard-coded in Go a list of extensions that will be enabled by default:

var builtinExtensionIDs = []string{
	"sourcegraph/typescript",
	"sourcegraph/python",
	"sourcegraph/java",
	"sourcegraph/go",
	"sourcegraph/cpp",
	"sourcegraph/ruby",
	"sourcegraph/php",
	"sourcegraph/csharp",
	"sourcegraph/shell",
	"sourcegraph/scala",
}

@dadlerj Although I asked you about DB migration earlier today, I did not end up creating one.

cc @ryan-blunden

TODO

• Add Default settings to the GraphQL schema
• Update docs to reflect that we expect language servers to be set up later and only if basic-code-intel is not sufficient
• Double check frontend code to see if it needs to be updated (e.g. browser extension)
• Merge https://github.com/sourcegraph/sourcegraph-typescript/pull/102
• Remove /nonexistent link