Add ability to disable masterbar from theme

Currently, to disable the `masterbar` module, you do so with the `jetpack_active_modules` filter.

Because of load order, this filter can only be run from a plugin and not a theme.

It would be helpful if you could disable the masterbar module from a theme.

Start by tracing the jetpack_active_modules filter and the masterbar module's load order, comparing how the filter is available from plugins versus themes. The change is complete when a theme can disable masterbar without relying on a plugin; verify the behavior in a WordPress theme and plugin context.

Written by the indexing model from the issue text.