* Fix login on Mastodon iOS app for users with no statuses
Mastodon for iOS can't cope with an empty string for a date and
expect a JSON `null` instead.
Fixes https://github.com/superseriousbusiness/gotosocial/issues/1010
* Fix expected values in tests to match
* validate web-asset-base-dir
* move default icons into converter
* always ensure avatar + header on api accounts
* update tests
* add default header
* don't return error from web module creation anymore
* tidy a bit
* use pngs for default avatars rather than svgs
* don't omitempty on description
* don't omitempty on any fields
* add ms to timestamp format
* don't omitempty on text_url
* rearrange attachment fields a bit
* just give URL again as attachment text url
* update tests
* fix accidental replace
