Add collation to directory listings (#1823)

* Add collation to directory listings. Closes #1806

Introduce a new `locale` arg to the `Query.directory` field. Set "en"
as the default for the field for backward compatibility. Use the given
locale, sending it through a language matcher, and use `x/text` as the
collation engine for the matched language.

Augment the file `ListDirs` call to optionally take a Collator. If the
Collator is given, sort file listings according to the collators rules.

While here, document the GraphQL schema a bit more.

Add matchers by looking at the current front-end locales, and make sure
each of these occur in the matcher list.

* Language matcher touchups

* Avoid having `en-US` twice.
* Introduce `en-AU`.

* Pass IgnoreCase and Numeric collation

Allow the collator to be configured with options. Pass the options
IgnoreCase and Numeric to the collator.

vendor/golang.org/x/text/collate/index.go

@@ -0,0 +1,32 @@
+// Copyright 2013 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package collate
+
+import "golang.org/x/text/internal/colltab"
+
+const blockSize = 64
+
+func getTable(t tableIndex) *colltab.Table {
+	return &colltab.Table{
+		Index: colltab.Trie{
+			Index0:  mainLookup[:][blockSize*t.lookupOffset:],
+			Values0: mainValues[:][blockSize*t.valuesOffset:],
+			Index:   mainLookup[:],
+			Values:  mainValues[:],
+		},
+		ExpandElem:     mainExpandElem[:],
+		ContractTries:  colltab.ContractTrieSet(mainCTEntries[:]),
+		ContractElem:   mainContractElem[:],
+		MaxContractLen: 18,
+		VariableTop:    varTop,
+	}
+}
+
+// tableIndex holds information for constructing a table
+// for a certain locale based on the main table.
+type tableIndex struct {
+	lookupOffset uint32
+	valuesOffset uint32
+}