[Monolog] added NotFoundActivationStrategy from MonologBundle

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

The NotFoundActivationStrategy class from MonologBundle is totally independent of the framework and should be part of the bridge instead. That would allow people to use it easily with Silex for instance.

ping @Seldaek

Maybe best to move this inline in the if, or at least return early if $isActivated is false, to avoid getting the request every time? Otherwise the PR and RequestStack changes seem fine to me.