Don't user root path for HAL endpoints

Update LinksMvcEndpoint and HalBrowserMvcEndpoint so that no longer try
to use the `/` context path. Links are now available from `/links` and
the HAL browser is available from `/hal`.

The actuator HAL browser now works with either WebJars or the Spring
Data version. It also now transforms the initial HTML so that the
form is pre-populated with `/links`.

When using Spring Data's HAL browser, the root includes a link to
`/links` with a rel of `actuator`.

See gh-3621

spring-boot-actuator-docs/src/restdoc/java/org/springframework/boot/actuate/hypermedia/HypermediaEndpointDocumentation.java

@@ -82,7 +82,7 @@ public class HypermediaEndpointDocumentation {
 
 	@Test
 	public void home() throws Exception {
-		this.mockMvc.perform(get("/").accept(MediaType.APPLICATION_JSON))
+		this.mockMvc.perform(get("/links").accept(MediaType.APPLICATION_JSON))
 				.andExpect(status().isOk()).andDo(document("admin"));
 	}