More options to manage subscriptions

[ElsaWeb]

Hello,

I have continued to work on Cachet during the past few weeks and you may be interested on these features.

This PR gives a finer grained control to subscribers to manage their subscriptions.

• Additionally to the component lists, a subscriber can choose to be notified to component updates (vs only incidents affecting the components).
• They can also choose to receive notifications about maintenances, whereas they would previously only receive them when being subscribed to all components (what was called 'globally').
• Finally, a subscriber can choose to be subscribed globally, i.e. receive notifications about incidents that are not affecting any components.

This follows #62 who should probably merged first, but if needed I may split them.

Thanks,