Conflicting plugins: improve the way we handle conflicting plugins

At the moment, we use 2 functions, `check_open_graph` ([#](https://github.com/Automattic/jetpack/blob/2.9.1/class.jetpack.php#L630)) and `check_twitter_tags` ([#](https://github.com/Automattic/jetpack/blob/2.9.1/class.jetpack.php#L702)) to check if a conflicting plugin is active, and deactivate the Open Graph Tags or the Twitter Cards if we find a conflicting plugin.

There are a few problems with our existing approach:
- Some of the code is duplicated in both of these functions
- If we disable Open Graph Tags, Twitter Cards also get disabled as well since they're hooked into the Open Graph tags ([#](https://github.com/Automattic/jetpack/blob/2.9.1/functions.twitter-cards.php#L133)) - [#2052-plugins](https://plugins.trac.wordpress.org/ticket/2052)
- A conflicting plugin can be activated while its Open Graph functions are not active. It would nice to detect that (#53).