Make it part of the info thing (and do a migration to ensure it's there)

Closes: #557

These attributes are documented as required by the Mastodon API.
Since we don’t support them (I think?), respond with default values.

Certain Mastodon/Pleroma front ends call verify_credentials to get the
default scope of a new toot.

Currently, Pleroma hardcodes this value to "public".

This patch changes it to the user's default_scope value.

This removes the placehold.it dependency.