Add Additional Fields for Issuer Registration

[dvorak42]

In order to clarify the uses of different issuers, we're considering updating the issuer registration process with some more explicit fields. Particularly, we're looking at adding the following fields to the registration process:

• Product Name - A human-readable name corresponding to the product using PST.
• Privacy Policy - A link to the relevant privacy policy surrounding use of the API.
• Explicit Purpose - A list of purposes that the issuer can choose from and could be surfaced in the JSON structure containing registered issues.

The purposes we are considering having are:

• Account safety
• Invalid Traffic Detection in Advertising
• Invalid Traffic Detection for non-advertising use cases
• Payment & e-commerce protections
• Bulk content abuse (e.g. Spam) prevention
• Other

Issuers that select "Other" will need to write down their purpose in the form, so we can update the list of explicit purposes over time to correspond to all registered purposes.