In this version we will support only relative paths in artifacts
metadata. Support for absolute paths will be introduced later.

This changes the format of metadata to handle paths, that may contain
whitespace characters, new line characters and non-UTF-8 characters.

Now those paths along with metadata in JSON format are stored as
length-prefixed strings (uint32 prefix).

Metadata file has a custom format:

1.   First string field is metadata version field (string)
2.   Second string field is metadata errors field (JSON strong)
3.   All subsequent fields is pair of path (string) and path metadata
     in JSON format.

Path's metadata contains all fields that where possible to extract from
ZIP archive like date of modification, CRC, compressed size,
uncompressed size and comment.

[ci skip]

Artifacts metadata field will be used to store a filename of gzipped
file containing metadata definition for given artifacts archive.

This is needed because of backward compatibility. Previously artifacts
archive had `.tar.gz` format, but artifacts browser requires ZIP format
now.

`artifacts?` method checks if artifacts archive is available.