Octopus free (power up) event support fixes

What's Changed

• Fix issue with manual API rates and decimal points by @springfall2008 in #1649
• Missing config element for free sessions by @springfall2008 in #1650
• Octopus free scraping from web site by @springfall2008 in #1651

Note: If the Octopus Free Session event sensor is not working due to lack of data (via a 3rd party service) Predbat can scrape directly from the Octopus Web Site, this may
have its own issues due to change of format. If you enable this then sessions will be considered even if you forget to sign-up so be careful!

octopus_free_url: 'http://octopus.energy/free-electricity'

Full Changelog: v8.7.1...v8.7.2