activitypub: actually send digest header when federating

See merge request !261

this is needed for backwards compatibility with non-digest pleroma instances

digest signature validation

See merge request !260

activitypub: verify remote http signature digests by recomputing the digest and replacing the digest header

Feature/filter exif (  solve #187 )

See merge request !237

Articles/Notes with a actor list outside object

See merge request !252

ostatus: return AS2 objects on /notice and /activities URLs like with /objects.

Closes #206

See merge request !251

serverside frontend configuration

See merge request !256

Feature/media description

Closes #174

See merge request !255

mastodon api: support descriptions in media api, add PUT endpoint for updating metadata about a media upload

Mastodon does not use the object name as alt text when the object is an Image.