fix: fix tailnet netcheck issues

[deansheather]

• Add STUN nodes to every Coder-managed DERP region (previously was only the primary region).
  • This means that regardless of which DERP region is selected, tailscale will always try to do STUN. Previously (AFAIK) if
• Avoid adding port to DERP URL in healthcheck if it's the default port for the URL scheme.
• Rework tailnet connection accept mechanism so we can update tailscale dependency.

Depends on coder/tailscale#28 and coder/tailscale#29 which:

• Fixes DERP healthcheck on non-standard HTTP(S) ports (in compliment with the second point above)
• Avoids using STUN latency as the indicator for the region latency. This means that HTTP(S) latency checks are always performed on every region.
• Fixes ForceHTTP nodes in netcheck