add link checker

[pfent]

No description provided.

[MoritzS]

The Slack URL only works with a non-Curl user-agent:

$ curl --http1.1 -H 'User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:151.0) Gecko/20100101 Firefox/151.0' -Lsv https://bonsai.cedardb.com/slack -o /dev/null 2>&1 | grep -iE 'HTTP|Host:|Location:'
* ALPN: curl offers http/1.1
* ALPN: server accepted http/1.1
* using HTTP/1.x
> GET /slack HTTP/1.1
> Host: bonsai.cedardb.com
< HTTP/1.1 301 Moved Permanently
< Location: https://join.slack.com/t/cedardbcommunity/shared_invite/zt-2r59cikfm-gF3LhvX~1Nv76MW7TsXHiA
* Issue another request to this URL: 'https://join.slack.com/t/cedardbcommunity/shared_invite/zt-2r59cikfm-gF3LhvX~1Nv76MW7TsXHiA'
* ALPN: curl offers http/1.1
* ALPN: server accepted http/1.1
* using HTTP/1.x
> GET /t/cedardbcommunity/shared_invite/zt-2r59cikfm-gF3LhvX~1Nv76MW7TsXHiA HTTP/1.1
> Host: join.slack.com
< HTTP/1.1 302 Found
< location: https://cedardbcommunity.slack.com/join/shared_invite/zt-2r59cikfm-gF3LhvX~1Nv76MW7TsXHiA
* Issue another request to this URL: 'https://cedardbcommunity.slack.com/join/shared_invite/zt-2r59cikfm-gF3LhvX~1Nv76MW7TsXHiA'
* ALPN: curl offers http/1.1
* ALPN: server accepted http/1.1
* using HTTP/1.x
> GET /join/shared_invite/zt-2r59cikfm-gF3LhvX~1Nv76MW7TsXHiA HTTP/1.1
> Host: cedardbcommunity.slack.com
< HTTP/1.1 200 OK

Does the link checker support this? If not, can we just whitelist/exclude slack?