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

helpers/language.go

@@ -140,6 +140,11 @@ func (l *Language) GetStringMapString(key string) map[string]string {
 	return cast.ToStringMapString(l.Get(key))
 }
 
+//  returns the value associated with the key as a slice of strings.
+func (l *Language) GetStringSlice(key string) []string {
+	return cast.ToStringSlice(l.Get(key))
+}
+
 // Get returns a value associated with the key relying on specified language.
 // Get is case-insensitive for a key.
 //