Rename taxonomy kinds from taxonomy to term, taxonomyTerm to taxonomy

And we have taken great measures to limit potential site breakage:

* For `disableKinds` and `outputs` we try to map from old to new values if possible, if not we print an ERROR that can be toggled off if not relevant.
* The layout lookup is mostly compatible with more options for the new `term` kind.

That leaves:

* Where queries in site.Pages using taxonomy/taxonomyTerm Kind values as filter.
* Other places where these kind value are used in the templates (classes etc.)

Fixes #6911
Fixes #7395

common/loggers/ignorableLogger.go

@@ -0,0 +1,52 @@
+// Copyright 2020 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 loggers
+
+import (
+	"fmt"
+	"strings"
+)
+
+// IgnorableLogger is a logger that ignores certain log statements.
+type IgnorableLogger struct {
+	logger     *Logger
+	statements map[string]bool
+}
+
+// NewIgnorableLogger wraps the given logger and ignores the log statement IDs given.
+func NewIgnorableLogger(logger *Logger, statements ...string) IgnorableLogger {
+	statementsSet := make(map[string]bool)
+	for _, s := range statements {
+		statementsSet[strings.ToLower(s)] = true
+
+	}
+	return IgnorableLogger{
+		logger:     logger,
+		statements: statementsSet,
+	}
+}
+
+func (l IgnorableLogger) Errorf(statementID, format string, v ...interface{}) {
+	if l.statements[statementID] {
+		// Ignore.
+		return
+	}
+	ignoreMsg := fmt.Sprintf(`
+If you feel that this should not be logged as an ERROR, you can ignore it by adding this to your site config:
+ignoreErrors = [%q]`, statementID)
+
+	format += ignoreMsg
+
+	l.logger.ERROR.Printf(format, v...)
+}