* Optimize map { ... }.compact

using Enumerable#filter_map, supported since Ruby 2.7

* Add poyfill for Enumerable#filter_map

This change is to suppress the warning below on on ruby-2.7.0:

- warning: Using the last argument as keyword parameters is deprecated; maybe ** should be added to the call

https://www.ruby-lang.org/en/news/2019/12/12/separation-of-positional-and-keyword-arguments-in-ruby-3-0/

Reject those from accounts with no local followers, from relays
that are not enabled, which do not address local accounts and are
not replies to accounts that do have local followers

* Do not LDS-sign Follow, Accept, Reject, Undo, Block

* Do not use LDS for Create activities of private toots

* Minor cleanup

* Ignore unsigned activities instead of misattributing them

* Use status.distributable? instead of querying visibility directly

* Fix handling of malformed ActivityPub payloads when URIs are nil

* Gracefully handle JSON-LD canonicalization failures

- Rename Mastodon::TimestampIds into Mastodon::Snowflake for clarity
- Skip for statuses coming from inbox, aka delivered in real-time
- Skip for statuses that claim to be from the future

* Don't process ActivityPub payload if signature is invalid

* Fix style issue

* Add handling of Linked Data Signatures in payloads

* Add a way to sign JSON, fix canonicalization of signature options

* Fix signatureValue encoding, send out signed JSON when distributing

* Add missing security context

* Add ActivityPub inbox

* Handle ActivityPub deletes

* Handle ActivityPub creates

* Handle ActivityPub announces

* Stubs for handling all activities that need to be handled

* Add ActivityPub actor resolving

* Handle conversation URI passing in ActivityPub

* Handle content language in ActivityPub

* Send accept header when fetching actor, handle JSON parse errors

* Test for ActivityPub::FetchRemoteAccountService

* Handle public key and icon/image when embedded/as array/as resolvable URI

* Implement ActivityPub::FetchRemoteStatusService

* Add stubs for more interactions

* Undo activities implemented

* Handle out of order activities

* Hook up ActivityPub to ResolveRemoteAccountService, handle
Update Account activities

* Add fragment IDs to all transient activity serializers

* Add tests and fixes

* Add stubs for missing tests

* Add more tests

* Add more tests