Add proper support for proxies

There are situations where you want tt-rss to use a proxy (e.g.
because of network restrictions, or privacy concerns).
tt-rss already comes with an undocumented `_CURL_HTTP_PROXY`
variable (see eg https://binfalse.de/2015/05/06/ttrss-with-proxy/),
however that won't have an effect when, for example, php-curl is
not installed, see
https://git.tt-rss.org/git/tt-rss/src/c30f5e18119d1935e8fe6d422053b127e8f4f1b3/include/functions.php#L377
In this case it would use the `file_get_contents` with a stream
context without a proxy definition:
https://git.tt-rss.org/git/tt-rss/src/c30f5e18119d1935e8fe6d422053b127e8f4f1b3/include/functions.php#L487

Here I propose to properly support proxies, and I introduced a
`PROXY` variable, that is respected in both scenarios, with and
without curl installed.