hgweb: add docFromHTML javascript function
It takes a string with HTML markup and creates DOM document from it.
The implementation is a part of code from
https://developer.mozilla.org/en-US/docs/Web/API/DOMParser#DOMParser_HTML_extension_for_other_browsers
hgweb: add makeRequest javascript function
This function performs an asynchronous HTTP request and calls provided
callbacks:
- onstart: request is sent
- onsuccess: response is received
- onerror: some error occured
- oncomplete: response is fully processed and all other callbacks finished