CORS proxy improvements

[hughbris]

I noticed the plugin's proxy URL was blocked by my webserver rules. It seemed like a problematic path anyway. So replaced the proxy path to /_cors/proxy by default (2b46aa3) and allowed it to be configured (662e9f3).

[wernerjoss]

Hello Hugh,
thanks for working on improvement of this project.
I have cloned your version to my local docker test instance and it works without problems, which is no wonder, as I'm not using any CORS proxy 😄
the Idea to provide a configurable proxy path sounds good, but, ATM, I can't test this as I do not have any more remote Calendars available .
then, I see that the formerly provided proxy, proxy.php, ist no more included, which does not seem convenient for people who do not know where to find one.
plus, what will happen to those who currently use the existing proxy.php after an update ?