Add migration to rewrite pending commands to use folder IDs

class StoreSchemaDefinition implements SchemaDefinition {

    static final int DB_VERSION = 72;

    static final int DB_VERSION = 73;

    private final MigrationsHelper migrationsHelper;

package com.fsck.k9.storage.migrations;

class LegacyPendingAppend extends LegacyPendingCommand {

    public String folder;

    public String uid;

}

package com.fsck.k9.storage.migrations;

abstract class LegacyPendingCommand {

}

import static com.fsck.k9.controller.Preconditions.requireValidUids;

import static com.fsck.k9.helper.Preconditions.checkNotNull;

class LegacyPendingDelete {

class LegacyPendingDelete extends LegacyPendingCommand {

    public final String folder;

    public final List<String> uids;

package com.fsck.k9.storage.migrations;

class LegacyPendingExpunge extends LegacyPendingCommand {

    public String folder;

}