Fix saving calculation (missing standard charge), tweaks

What's Changed

Saving calculations were missing the standing charge

This made the figures rather pessimistic if you had standing charge enabled.

• Fix missing standing charge in savings figures, put total in £s by @springfall2008 in #1112
• Fix cost saving chart (should be offset 6 days) by @springfall2008 in #1102

Tweaks

• Remove fast plan mode support (and related code)

• Change combine_charge_slots option to off by default and updated the documentation to reflect this

• Changed the default charge slot split to 30 minutes

• Optimisation tweaks by @springfall2008 in #1109

Documentation updates

• Documentation: predbat plan, inverter time FAQ, entity too large warnings by @gcoan in #1103

Full Changelog: v7.19.1...v17.19.2