server_name is not correctly validated for join requests

This issue has been migrated from [#7108](https://github.com/matrix-org/synapse/issues/7108).

---

### 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 ((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- 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

Start with the logged client join request and trace how the server_name query parameter is parsed and validated during federation make_join handling. The issue provides no reproduction steps or named files/tests; done should mean malformed server_name values are rejected cleanly rather than causing the shown ValueError and 502 failure.

Written by the indexing model from the issue text.