Unblock of a blocked GtS account from another instance receives a 403 forbidden

[tsmethurst]

Essentially, the unblock activity is being refused, because of the existing block.

To reproduce: block a gotosocial account from another instance. Then unblock it. The block will go through but the unblock will produce a 403.

We should add an exception where 'undo' activity on a block skips through the block checking logic, otherwise a blocked user can never be unblocked again.

logs:

time="2021-09-23T12:24:56Z" level="debug" msg="entering BLOCKED function with IRI list: [https://ondergrond.org/users/dumpsterqueer]" func="github.com/superseriousbusiness/gotosocial/internal/federation.(*federator).Blocked" file="github.com/superseriousbusiness/gotosocial/internal/federation/federatingprotocol.go:183" fields.func="Blocked" func="github.com/superseriousbusiness/gotosocial/internal/federation.(*federator).Blocked"
time="2021-09-23T12:24:56Z" level="trace" msg="SELECT count(*) FROM \"domain_blocks\" AS \"domain_block\" WHERE (LOWER(domain) = LOWER('ondergrond.org'))" func="github.com/superseriousbusiness/gotosocial/internal/db/bundb.(*debugQueryHook).AfterQuery" file="github.com/superseriousbusiness/gotosocial/internal/db/bundb/trace.go:52" operation="SELECT" queryTime="1.454ms"
time="2021-09-23T12:24:56Z" level="trace" msg="SELECT count(*) FROM \"blocks\" AS \"block\" WHERE (account_id = '01JNN207W98SGG3CBJ76R5MVDN') AND (target_account_id = '00ZM91JAGCPDQVDW6XS71BYP59') OR (target_account_id = '01JNN207W98SGG3CBJ76R5MVDN') AND (account_id = '00ZM91JAGCPDQVDW6XS71BYP59')" func="github.com/superseriousbusiness/gotosocial/internal/db/bundb.(*debugQueryHook).AfterQuery" file="github.com/superseriousbusiness/gotosocial/internal/db/bundb/trace.go:52" operation="SELECT" queryTime="766µs"
time="2021-09-23T12:24:56Z" level="info" msg="Forbidden: wrote -1 bytes in 28.81653ms" func="github.com/superseriousbusiness/gotosocial/internal/router.loggerWithConfig.func1" file="github.com/superseriousbusiness/gotosocial/internal/router/logger.go:65" clientIP="192.119.111.147" latency="28.81653ms" method="POST" path="/users/dumpsterqueer/inbox" statusCode="403" userAgent="http.rb/4.4.1 (Mastodon/3.3.0; +https://ondergrond.org/)"