Add link to open any port in dashboard

[presleyp]

Once #3753 is merged, users can navigate to ports running on their agent from the web browser 🌏 🔗

For example, if I have a Python server running on port 3333 on my main agent in my blue workspace on dev.coder.com, I can access the app via: https://bpmct--blue--main--3333.dev.coder.com/ OR https://dev.coder.com/@bpmct/blue/apps/3333/.

Prior art

@code-asher created a port-forwarding dropdown in the workspaces that also auto-detects ports via agent: #1824. This was previously blocked by no backend routing (#3753).

TODO

• There are some todos in feat: port forwarding dropdown #1824
• If an admin doesn't set up a wildcard domain, the port forwarding dropdown should not use a subdomain.
  • Coder Classic entirely disabled port forwarding via the dashboard when coderd. devurlsHost was not set.
  • @bpmct suggestion: use a deployment-wide environment variable to enable subdomain apps. When disabled, use path URLs and link to docs to enable subdomains via the dropdown (many apps don't work by default as a path).

[BrunoQuaresma]

Done in #3812

[ammario]

@deansheather is taking another stab at this.

[kylecarbs]

This is now done!