Deprecate the Http class

Deprecate the Http class

All methods got moved to HttpRequestFactory or MWHttpRequest or dropped.

I made the return value of the new HttpRequestFactory::request/get/post
methods null on error instead of false, so that when we drop PHP 7
support, we can use a "?string" return value. This could theoretically
change behavior of code that was switched from the old Http methods, but
probably won't. I kept the old behavior for the deprecated methods.

I changed the default value of $wgHTTPProxy from false to ''. This way
it should be usable directly without a trivial wrapper method. For the
benefit of anyone who might have set it to false in LocalSettings.php, I
also recommend casting to string just in case.

Http::$httpEngine is deprecated. Eventually it will be removed along
with the curl and PHP engines, leaving only the Guzlle engine.

I also added deprecation of MWHttpRequest::factory, which occurred in
1.31, to the release notes for 1.34. Now hopefully we can hard-deprecate
it in another couple of versions.

Bug: T214390
Change-Id: I2a316a758d793857f248bd251b90f5e9a6440e3a