Stalwart refuses a blobId in a card's media ("blobIds in media is not
supported"), so adding or changing a photo always failed. The editor now
saves the photo as a data: URI, which Stalwart accepts and returns
unchanged, and leaves the card's other media as it was. Checked live on
0.16.22; the mock now refuses a blobId the same way.
Avatars in the mail list come from the address book's cards, and nothing
loaded those at sign-in, so a photo showed only after Contacts had been
opened. The cards now load in the background at start, the avatar uses
whatever cards are held, and a shared card's photo is fetched from the
account it belongs to.
Fixes#376.