gotosocial has alt-text for avatars and banners, we should too

apparently GTS allows you to set alt-text for your avatar and your profile banner

it currently (0.17.3) doesn't seem to expose the text in the AP representation of the profile, but once they do, we should do the same (I guess it'd be a summary inside the icon or banner objects?)

see https://gts.social.violetmillie.me/@millie/statuses/01JKKCXF7GS1P7ZTDTAAPQBK7M and https://github.com/superseriousbusiness/gotosocial/issues/3450

added enhancement label

changed the description

added federation label

if I remember correctly, chuckya also added support for this. I wonder if their implementation differs from gotosocial. https://github.com/TheEssem/mastodon/commit/bdc093db0221b4df1ceb0c9c110ec5e71845d3a0