Add optional welcome message.

See merge request !832

oauth: never use base64 padding when returning tokens to applications

See merge request !825

Hellthread filter fix

Closes #634

See merge request pleroma/pleroma!831

activitypub: correctly handle bare URIs in user lookup

Closes #645

See merge request !829

get_or_fetch() handles the nickname verses URI differences transparently.

activitypub transmogrifier: fix reserialization edge cases

See merge request !826

Fix S3 links encoding in Mediaproxy

Closes #580

See merge request !828

Fix credo issues in ./test

See merge request !808

activitypub: user inbox/outbox: `totalItems` removal

See merge request !827

(this is based on a counter in User.Info, but the counter is not reliable.)

It is not really feasible to quickly calculate the totalItems value and
it shouldn't be trusted anyway.

ActivityPub C2S oauth endpoints

See merge request !821

The normal Base64 alphabet uses the equals sign (=) as a padding character.  Since
Base64 strings are self-synchronizing, padding characters are unnecessary, so don't
generate them in the first place.

Improve login error for OAuth flow

See merge request !824