Issue checklist for v2.1.0

[schlessera]

Checklist of open issues we want to include for release v2.1.0:

wp-cli/db-command

• --size_format not working correct for DB size

wp-cli/entity-command

• wp post generate --post_content hangs if there's no STDIN (does not work within Behat)

wp-cli/extension-command

• plugin update does not fail on bad file permissions
• wp theme mod get --all produces malformed output for non-tabular formats (punted)

wp-cli/i18n-command

• Invalid MD5 hashes in make-json command (invalid)

wp-cli/language-command

• PHP notice on updating theme translations with no updates
• Allow to install language packs for all plugins

wp-cli/scaffold-command

• Block scaffold command produces a syntax error when used with --theme argument

wp-cli/wp-cli

• --prompt should skip flags that have been included in the command (to be discussed)
• WP-CLI does not use the same way as wordpress to locate the wp-config.php
• Packages cannot override bundled commands in Phar anymore
• WP CLI can no longer be installed on Windows
• Bump PHP version check in bootstrap code to v5.4+

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Fixes #41