Incremental improvements on security advisory form

We've recently released a few minor user experience improvements for our GitHub Security Advisory form:

• You're no longer required to fill out as many fields in the form before submitting it, so you can publish faster.
• You now fill out title/description first in the form.
• You can now access the CVSS Calculator as a top-level attribute, rather than it being the bottom of a dropdown menu.

Further reading:

• Learn more about GitHub Security Advisories
• Learn more about the GitHub Advisory Database