client/web: always use new web client; remove old client

This uses the new react-based web client for all builds, not just with
the --dev flag.

If the web client assets have not been built, the client will serve a
message that Tailscale was built without the web client, and link to
build instructions. Because we will include the web client in all of our
builds, this should only be seen by developers or users building from
source. (And eventually this will be replaced by attempting to download
needed assets as runtime.)

We do now checkin the build/index.html file, which serves the error
message when assets are unavailable.  This will also eventually be used
to trigger in CI when new assets should be built and uploaded to a
well-known location.

Updates tailscale/corp#13775

Signed-off-by: Will Norris <will@tailscale.com>

README.md

@@ -57,6 +57,17 @@ If your distro has conventions that preclude the use of
 `build_dist.sh`, please do the equivalent of what it does in your
 distro's way, so that bug reports contain useful version information.
 
+## Building the web client
+
+To include the embedded web client (accessed via the `tailscale web` command),
+you'll need to build the client assets using:
+
+```
+./tool/yarn --cwd client/web build
+```
+
+Do this before building the `tailscale.com/cmd/tailscale` binary.
+
 ## Bugs
 
 Please file any issues about this code or the hosted service on