Add a way to disable one or more languages

This commit adds a new config setting:

```toml
disableLanguages = ["fr"]
```

If this is a multilingual site:

* No site for the French language will be created
* French content pages will be ignored/not read
* The French language configuration (menus etc.) will also be ignored

This makes it possible to start translating new languages and turn it on when you're happy etc.

Fixes #4297
Fixed #4329

config/configProvider.go

@@ -20,6 +20,7 @@ type Provider interface {
 	GetBool(key string) bool
 	GetStringMap(key string) map[string]interface{}
 	GetStringMapString(key string) map[string]string
+	GetStringSlice(key string) []string
 	Get(key string) interface{}
 	Set(key string, value interface{})
 	IsSet(key string) bool