Modify Request::getClientIp() to use IpUtils::checkIp()

Adds the ability to use CIDR notation in the trusted proxy list
Allow setting trusted_proxies using CIDR notation
[HttpFoundation] added some unit tests for ranges of trusted IP addresses
[HttpFoundation] refactored code to avoid code duplication
[HttpFoundation] updated CHANGELOG
merged branch fabpot/trusted-proxies (PR #7735)

This PR was merged into the master branch.

Discussion
----------

[HttpFoundation][FrameworkBundle] Add CIDR notation support in trusted proxy list

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7312, #7262
| License       | MIT
| Doc PR        | symfony/symfony-docs#2287

Should be rebased once #7734 is merged.

Commits
-------

7b32794 [HttpFoundation] updated CHANGELOG
e7c1696 [HttpFoundation] refactored code to avoid code duplication
1695067 [HttpFoundation] added some unit tests for ranges of trusted IP addresses
811434f Allow setting trusted_proxies using CIDR notation
ddc9e38 Modify Request::getClientIp() to use IpUtils::checkIp()