Always verify TLS unless explicitly told otherwise

TLS was not being verified in a number of places:

- connections to grafana.com

- connections to OAuth providers when TLS client authentication was
  enabled

- connections to self-hosted Grafana installations when using the CLI
  tool

TLS should always be verified unless the user explicitly enables an
option to skip verification.

Removes some instances where `InsecureSkipVerify` is explicitly set to
`false`, the default, to help avoid confusion and make it more difficult
to regress on this fix by accident.

Adds a `--insecure` flag to `grafana-cli` to skip TLS verification.

Adds a `tls_skip_verify_insecure` setting for OAuth.

Adds a `app_tls_skip_verify_insecure` setting under a new `[plugins]`
section.

I'm not super happy with the way the global setting is used by
`pkg/api/app_routes.go` but that seems to be the existing pattern used.

pkg/api/grafana_com_proxy.go

@@ -1,7 +1,6 @@
 package api
 
 import (
-	"crypto/tls"
 	"net"
 	"net/http"
 	"net/http/httputil"
@@ -14,8 +13,7 @@ import (
 )
 
 var grafanaComProxyTransport = &http.Transport{
-	TLSClientConfig: &tls.Config{InsecureSkipVerify: false},
-	Proxy:           http.ProxyFromEnvironment,
+	Proxy: http.ProxyFromEnvironment,
 	Dial: (&net.Dialer{
 		Timeout:   30 * time.Second,
 		KeepAlive: 30 * time.Second,