Enable *opt-in* Mixpanel sign up

[vpetersson]

If the user opt-in, we want to be able to push exclusive offers via Mixpanel. Here's how this will be done.

Add a new settings field

We need to add a new line to screenly.conf that is named offer_opt_in (boolean). This line should be absent in the default config, and only be populated by user action.

First visit

When the user first visits any page without this setting line present, a pop-over should appear that asks the following question: `Would you like to receive news and exclusive offers from Screenly?'

If the user replies 'Yes', we set offer_opt_in to True.

Display new option on the settings page

Once the offer_opt_in is present in screenly.conf, we should expose this as a boolean on the settings page.

Conditional loading

If, and only if, the user have opted in for offers, we load the Mixpanel library with Web in-app notifications.

[stale[bot]]

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