Implement trip notifications and read state

[buyolitsez]

To close #340

• notify nearby hosts when a member creates a new public trip
• add a trip notification preference during signup/preferences
• allow hosts to hide trips from their visitors lists
• mark matching trip notifications as checked when a trip is hidden

[thisismeonmounteverest]

@buyolitsez Looks good. There are a few problems, though.

The notification needs to be on the subtrip as the reason for 'reading' a trip might be either the person it self or the time of the visit.

The notification tab is a left over from far, far away and will be removed in the near future so we do not need to worry about that anymore.

The original discussion was to allow for digests: Immediately, daily, weekly, monthly. Starting with immediately is fine for me, but with that in mind the code might need some tinkling.

To find the members to be informed we should make use of the new spatial indices for geonames, shouldn't we?

Do we want to allow members to 'read' a leg on the visitors page (/visitors) or only on the landing page?