Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

lazy/once.go

@@ -0,0 +1,69 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package lazy
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+// onceMore is similar to sync.Once.
+//
+// Additional features are:
+// * it can be reset, so the action can be repeated if needed
+// * it has methods to check if it's done or in progress
+//
+type onceMore struct {
+	mu   sync.Mutex
+	lock uint32
+	done uint32
+}
+
+func (t *onceMore) Do(f func()) {
+	if atomic.LoadUint32(&t.done) == 1 {
+		return
+	}
+
+	// f may call this Do and we would get a deadlock.
+	locked := atomic.CompareAndSwapUint32(&t.lock, 0, 1)
+	if !locked {
+		return
+	}
+	defer atomic.StoreUint32(&t.lock, 0)
+
+	t.mu.Lock()
+	defer t.mu.Unlock()
+
+	// Double check
+	if t.done == 1 {
+		return
+	}
+	defer atomic.StoreUint32(&t.done, 1)
+	f()
+
+}
+
+func (t *onceMore) InProgress() bool {
+	return atomic.LoadUint32(&t.lock) == 1
+}
+
+func (t *onceMore) Done() bool {
+	return atomic.LoadUint32(&t.done) == 1
+}
+
+func (t *onceMore) ResetWithLock() *sync.Mutex {
+	t.mu.Lock()
+	defer atomic.StoreUint32(&t.done, 0)
+	return &t.mu
+}