Add options page for tampering with Jetpack's constants

[oskosk]

Proposal for addressing Automattic/jurassic.ninja#125

Changes introduced by this Pull Request

It will add a Settings -> Jetpack Constants to every launched site if Jetpack is at least active on the site.

Testing instructions

1. Launch a regular JN site
2. ssh to it, step into the public dir and install the version from this branch

    cd  ~/apps/$USER/public
    wp plugin install --force --activate https://github.com/Automattic/companion/archive/add/jetpack-constants.zip

3. Visit wp-admin -> Settings and confirm that you can access the settings page this PR is introduceing under the Settings -> Jetpack Constants menu item.

[simison]

I tested this and it works brilliantly 👍