server_name is not correctly validated for join requests

[matrixbot]

This issue has been migrated from #7108.

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Description

Caused #4891 but is a problem by itself, too.

2020-03-19 16:02:39,463 - synapse.http.matrixfederationclient - 491 - WARNING - POST-591111- {GET-O-12659} [!NEdKsYgzazXzpqqeBT:grin.hu] Request failed: GET matrix://!NEdKsYgzazXzpqqeBT:grin.hu/_matrix/federation/v1/make_join/%21NEdKsYgzazXzpqqeBT%3Agrin.hu/%40grin%3Agrin.hu?ver=1&ver=2&ver=3&ver=4&ver=5&ver=org.matrix.msc2260: ValueError("invalid literal for int() with base 10: b'grin.hu'")

2020-03-19 16:02:39,463 - synapse.util.retryutils - 218 - INFO - POST-591111- Connection to !NEdKsYgzazXzpqqeBT:grin.hu was unsuccessful (<class 'synapse.api.errors.RequestSendFailed'>(Failed to send request: ValueError: invalid literal for int() with base 10: b'grin.hu')); backoff now 600000

2020-03-19 16:02:39,464 - synapse.federation.federation_client - 421 - WARNING - POST-591111- Failed to make_join via !NEdKsYgzazXzpqqeBT:grin.hu

2020-03-19 16:02:39,465 - synapse.http.server - 81 - INFO - POST-591111- <XForwardedForRequest at 0x7f49810bc128 method='POST' uri='/_matrix/client/r0/join/!NEdKsYgzazXzpqqeBT%3Agrin.hu?server_name=%21NEdKsYgzazXzpqqeBT%3Agrin.hu' clientproto='HTTP/1.1' site=8008> SynapseError: 502 - Failed to make_join via any server

Steps to reproduce

I don't know, I only became aware of this when it completely broke communities.

Version information

• Homeserver:
  matrix.grin.hu

• Version:
  v1.11.1-1 (debian)

• Install method:
  debian experimental repo.

• Platform:
  Debian/Sid/x64