Add compatibility with other Linked Signature algorithms

scripts/travis.sh

server.ts

server/helpers/custom-jsonld-signature.ts

server/helpers/peertube-crypto.ts

server/initializers/constants.ts

...er/lib/job-queue/handlers/utils/activitypub-http-utils.ts

server/middlewares/activitypub.ts

server/tests/api/activitypub/client.ts

server/tests/api/activitypub/helpers.ts

server/tests/api/activitypub/index.ts

...pi/activitypub/json/mastodon/bad-body-http-signature.json

...sts/api/activitypub/json/mastodon/bad-http-signature.json

...r/tests/api/activitypub/json/mastodon/bad-public-key.json

...s/api/activitypub/json/mastodon/create-bad-signature.json

server/tests/api/activitypub/json/mastodon/create.json

...r/tests/api/activitypub/json/mastodon/http-signature.json

server/tests/api/activitypub/json/mastodon/public-key.json

...i/activitypub/json/peertube/announce-without-context.json

server/tests/api/activitypub/json/peertube/invalid-keys.json

server/tests/api/activitypub/json/peertube/keys.json