FLAG-1014: Change main subscribe form and PTW to use Ortto instead of Pardot

[SARodrigues]

Overview

In this PR:

• Add/change/reorder fields in the /subscribe form
• interests is now sent as an array of interests, instead of a string with comma separated positions
• Updated the form handler link to the new one
• Updated message displayed after the user successfully submits the form
• Added the possibility to add descriptions to the select input
• Removed an unused pardot extra field
• The "Places to Watch" link now points to this subscribe form

Feature relevant tickets

FLAG-1014

[wri7tno]

code LGTM, untested