Update Download File URL to support HTTPS

[zfi]

The blocklyprop.properties file contains a single definition to indicate the location of the BlocklyProp Client download files.

downloadfiles.baseurl = http://blockly.example.com/download

When the site is accessed via HTTPS, this link will cause modern browsers to warn the user that there may be insecure content on the page. There are at least two options to resolve to resolve this issue

1. Remove the protocol portion of the URL in the configuration file and allow the application to figure out which protocol to use.

2. Add a second configuration option in the blocklyprop.properties file for each protocol.

downloadfiles.baseurl = http://blockly.example.com/download
downloadfiles.baseurl.https = https://blockly.example.com/download

Suggestions are appreciated.