[feature/symfony-3] Cloudflare IPs remain authorized after uninstalling the package

[KorvinSzanto]

Let's adjust the feature/symfony-3 branch to work like this:

From @mlocati in #5

• Usage

  1. concrete5 boots
  2. The request is told to trust the IPs stored in the concrete.security.trusted_proxies.ips configuration key

• IP update/configuration (cf:ip:update command and/or Update Cloudflare IPs job)

  1. read the contents of a new cloudflare_proxy::ips.last_fetched config key (see later) (call it PreviousCloudFlare)
  2. fetch the CloudFlare IP addresses (call it NewCloudFlare)
  3. read the contents of concrete.security.trusted_proxies.ips configuration key (call it CurrentlyTrusted)
  4. from this last list, we remove the IPs read at point 1, and we add the IPs fetched at point 2 (call it NewlyTrusted)
  5. store NewlyTrusted in the concrete.security.trusted_proxies.ips configuration key
  6. store NewCloudFlare in the cloudflare_proxy::ips.last_fetched configuration key

• After package uninstallation

  1. we remove from cloudflare_proxy::ips.last_fetched the IPs present in cloudflare_proxy::ips.last_fetched

With this approach, in addition to the previous pros of this pull request, we'll:

• let users to add their trusted IP addresses by using the default concrete.security.trusted_proxies.ips configuration key (for example, via the GUI implemented in Add a dashboard page to configure trusted proxy IPs concretecms#6703)
• have clean list of trusted IP addresses when the package is uninstalled (that is, Cloudflare IPs won't be trusted anymore)

[mlocati]

Fix implemented in #7