admin panel improvements

[ryanckulp]

the Speedrail default admin panel assumes Users are the focus, but here Bookings is the key model.

as an Admin, i want to see:

• upcoming Bookings + guest (User) contact info
• pricing GUI to modify per-night pricing
• availability GUI to block off dates that should not be sold

[ryanckulp]

cc @allancodenet if you want to take a look at this. we'll use ActiveAdmin for everything, and we could use a new table to store pricing. but instead of "Pricings" we could do something more generic like "Settings," which could be used across the app for miscellaneous stuff.

for example:

Setting
- key (string)
- value (string)

# examples
Setting.new(key: "per_night_price", value: "99.0")
Setting.new(key: "per_week_price", value: "499.0")

but for hiding dates we could probably just use our existing Bookings table. if a Booking belongs to the Admin User, it's simply blocked off. to end users, this is a distinctiong without a difference. a booked date is a blocked date.