* Add support for base64-encoded hash values
The global template function 'get_file_hash' can now return a
base64-encoded hash value when its 'base64' parameter is set to true.
See discussion in #519.
* Fix integrity attribute's value in test site
SRI hash values must be base64-encoded.
* Update documentation about 'get_file_hash'
* Fix 'can_get_hash_for_static_files' unit test