Ease local development, use PHP 7.4, various fixes

[arogachev]

Fixed issues:

• English is processed twice during make download #966
• Upgrade PHP version on server #960
• documentation generation is broken #828
• extension docs are not updated #500
• generating extension api docs does not work with PHP 7.1 syntax #331
• Extension search gives too many results of a single extension #664
• Search on Wiki is broken #407

TODO list (required):

• make guide-1.0
• make guide-1.1
• make guide-2.0
• make api-1.0
• make api-1.1
• make api-2.0
• make download-2.0
• ./yii search/rebuild
• ./yii contributors/generate (needs to be called twice, acceptable now, see "MySQL server has gone away" error during building contributors page locally #964).
• Add info about generating Yii 1.0 API docs to README.
• Add custom vendor dir to other places (tests, etc.).

TODO list (optional):

• Use Docker for generating Yii 1.0 API docs.
• Handle ./init in shared volume.
• Sync vendor and node_modules with host.
• Setup and configure debug environment.
• Add details of changes to this PR's description.
• Add extensions to Composer require section.

make guide-2.0

PDF errors in the following logs:

• data/guide-2.0/ru/pdf/fail.log
• data/guide-2.0/fr/pdf/fail.log
• data/guide-2.0/zh-cn/pdf/fail.log
• data/guide-2.0/ja/pdf/fail.log
• data/guide-2.0/pl/pdf/fail.log

make download-2.0

Building guide and api package for language:

• (uk) Error: DOMDocument::loadHTML(): Tag route invalid in Entity, line: 76
• (ar) Error: DOMDocument::loadHTML(): Unexpected end tag : p in Entity, line: 18
• (ja) Error: DOMDocument::loadHTML(): ID date-number already defined in Entity, line: 552
• (zh-CN) Error: DOMDocument::loadHTML(): ID creating-activeform-dropdownlist already defined in Entity, line: 127
• Broken encoding (for example, uk)