Custom install command for Hugo

This release adds the ability to configure a custom install command for Hugo sites – e.g. if you're using a package manager other than npm. This release also includes several other fixes and improvements.

Features:

• The install command for Hugo sites is now configurable.
• The "Urban" Bookshop starter template is now available for Hugo.

Fixes:

• The working directory is now reset after a custom build command, for consistency with other build hooks.
• Prevent builds crashing when the output directory has been configured with leading or trailing whitespace.
• Patched some security vulnerabilities.
• Deleting nested snippets no longer invalidates the parent snippet.
• Avoid request timeouts when opening a Cloudinary DAM in the CloudCannon asset browser.
• Fixed an issue preventing boolean keys displaying properly in card preview metadata.
• Migrating Forestry configuration to CloudCannon now adds remove_extra_inputs: false and hide_extra_inputs: true to schemas, to more closely match Forestry's behaviour for templates.