[Core API] Default signature & signature styling

We've released two changes to improve signature management in the Core API.

Set a default signature

A default signature can now be designated with any of the following endpoints:

To set the new/updated signature as the default, include is_default: true in the request body. is_default is now also exposed in all signature responses to reveal whether a given signature will be selected by default when composing a message.

:warning: Note: previously, all signatures created through the API had equal priority; it's possible that multiple signatures simultaneously show is_default: true. This is safe to ignore, but it can also be addressed case-by-case: when a signature is created or updated with is_default: true, all signatures for that team or teammate will be re-sorted.

Signature styling

Signatures now support a broader set of HTML elements and inline styling.