SSL Verify Peer

[w00fz]

Overview

In some occasions we've noticed people unable to connect to GPM. After further investigations it turns out that on some systems (Windows only has been confirmed so far), the SSL certificate cannot be verified, causing GPM to fail (references: #1044, #1008 and potentially #850).
The to go solution is to disable the peer verification from cURL and fopen.

Purpose

This PR adds 2 new optional system config: system.gpm.verify_peer (default: true) and system.gpm.method (default: auto).

system.gpm.verify_peer

When enabled, the SSL certificate gets verified by either fopen or cURL methods. If disabled the checks is skipped (this is not advised but might be the only viable solution for some people)

system.gpm.method

By default is set to auto. Auto automatically tests if the fopen mechanism is available and if it is gets used. Otherwise it tries to see if cURL is available and if it is use that instead.

Usually auto works fine but there might be some instances where some might want to force either methods. This new setting will allow to do so.

Notes

• As long as the changes reported above, this PR also makes some code rearrangement for the Proxy, it should just work the same but might be good to get this reviewed/tested.
• This PR also introduces settings for Admin, new lang have been added with getgrav/grav-plugin-admin@0a2b1d7.

[rhukster]

Looks good to me!

[w00fz]

Missing code ref. c4f71c9