SDC - Update block names in CivicTheme and civictheme_subtheme_starter kit

Problem/Motivation

As per SDC spec, the name of the block should be different from the name of the variable used within this block. So we need a unified way to name these block's without constantly inventing new names.

Adding _slot suffix is the easiest approach. It does not have to be _slot - can be anything else - but has to be different from the variable name.

Proposed resolution

1. Add `_slot` suffix to civictheme and starter kit blocks.
2. Update upgrade script toupdate sub-theme block names and references to CT blocks.