Search: exclude "media" post type by default

#### Is your feature request related to a problem? Please describe.
The "media" post type is enabled by default on sites with Jetpack Search. Results of this type are often not very useful:

#### Describe the solution you'd like
The post type "media" is turned off by default. Users can turn it back on using the 'excluded post types' configuration in Customizer.

#### Additional context
We noticed this on the Automattic Field Guide. The first relevant result was in 3rd position, when it would have been at the top with the media post type excluded.

cc @jsnmoon who suggested this idea.

No files or tests are named. Start by locating the Jetpack Search configuration and the Customizer's excluded post types setting; verify how default post types are selected. Done means media is excluded by default while users can still re-enable it through that configuration, with relevant tests updated or added if they exist.

Written by the indexing model from the issue text.