Install / Update should respect wp_config constants

[Luke Nelson]

There are two constants in wp_config.php that disallow file editing, DISALLOW_FILE_EDIT and DISALLOW_FILE_MODS. Specifically, DISALLOW_FILE_MODS is intended to block theme and plugin updates, it even hides the update icon in the admin bar.

Simpods should do what WordPress does and honor the setting by not updating if the constant is defined.

Reference:

https://wordpress.org/support/article/editing-wp-config-php/#disable-the-plugin-and-theme-editor

https://wordpress.org/support/article/editing-wp-config-php/#disable-plugin-and-theme-update-and-installation

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[Jamin Xu]

Implemented DISALLOW_FILE_MODS