Revert to disregarding the mount path for /livereload/ endpoint (#2804)

Previously there was a pull request "Move livereload endpoint under the mount path (#2740)"
and it definitely makes sense as a way to make livereload work behind a proxy.

However, this change also caused a major annoyance when working on several different sites.
Steps to reproduce:

1. `serve` one site (needs to have `site_url` customized)
2. open the site in a browser
3. stop serving it but keep the browser
4. `serve` another site with a different mount point
5. observe spam of "404" errors in the new terminal (also in the old browser but that's whatever)

So, this reverts the observable effect of commit fafdcc2

mkdocs/livereload/__init__.py

@@ -32,7 +32,7 @@
             setTimeout(launchNext, 3000);
         }
     };
-    req.open("GET", "${mount_path}livereload/" + epoch + "/" + requestId);
+    req.open("GET", "/livereload/" + epoch + "/" + requestId);
     req.send();
 
     console.log('Enabled live reload');
@@ -201,10 +201,8 @@ def _serve_request(self, environ, start_response):
         # https://github.com/bottlepy/bottle/blob/f9b1849db4/bottle.py#L984
         path = environ["PATH_INFO"].encode("latin-1").decode("utf-8", "ignore")
 
-        if path.startswith(self.mount_path):
-            rel_file_path = path[len(self.mount_path):]
-
-            m = re.fullmatch(r"livereload/([0-9]+)/[0-9]+", rel_file_path)
+        if path.startswith("/livereload/"):
+            m = re.fullmatch(r"/livereload/([0-9]+)/[0-9]+", path)
             if m:
                 epoch = int(m[1])
                 start_response("200 OK", [("Content-Type", "text/plain")])
@@ -220,6 +218,9 @@ def condition():
                         self._epoch_cond.wait_for(condition, timeout=self.poll_response_timeout)
                     return [b"%d" % self._visible_epoch]
 
+        if path.startswith(self.mount_path):
+            rel_file_path = path[len(self.mount_path):]
+
             if path.endswith("/"):
                 rel_file_path += "index.html"
             # Prevent directory traversal - normalize the path.
@@ -266,9 +267,7 @@ def _inject_js_into_html(self, content, epoch):
             body_end = len(content)
         # The page will reload if the livereload poller returns a newer epoch than what it knows.
         # The other timestamp becomes just a unique identifier for the initiating page.
-        script = _SCRIPT_TEMPLATE.substitute(
-            mount_path=self.mount_path, epoch=epoch, request_id=_timestamp()
-        )
+        script = _SCRIPT_TEMPLATE.substitute(epoch=epoch, request_id=_timestamp())
         return b"%b<script>%b</script>%b" % (
             content[:body_end],
             script.encode(),

mkdocs/tests/livereload_tests.py

@@ -343,9 +343,9 @@ def test_serves_polling_instantly(self, site_dir):
             self.assertTrue(output.isdigit())
 
     @tempdir()
-    def test_serves_polling_from_mount_path(self, site_dir):
+    def test_serves_polling_with_mount_path(self, site_dir):
         with testing_server(site_dir, mount_path="/test/f*o") as server:
-            _, output = do_request(server, "GET /test/f*o/livereload/0/0")
+            _, output = do_request(server, "GET /livereload/0/0")
             self.assertTrue(output.isdigit())
 
     @tempdir()