Consider defaulting to AllowedResourceAliasChecker instead of SymlinkAllowedResourceAliasChecker

Currently we default to using `SymlinkAllowedResourceAliasChecker` in `ContextHandler`.
```java
if (File.separatorChar == '/')
addAliasCheck(new SymlinkAllowedResourceAliasChecker(this));
```

We should consider using `AllowedResourceAliasChecker` as a safer default, as `SymlinkAllowedResourceAliasChecker` will allow symlinks to be followed outside of the web root directory.

This is an intentional feature of `SymlinkAllowedResourceAliasChecker` so if we change the default we should not do it until a major release as users may be depending on this behavior.