Add entities to support per-device and per-scanner options for ref_power and to exclude proxies from calculations

[agittins]

TL/DR:

• Allow certain proxies (or local bluetooth adaptors) to be excluded from Bermuda's calculations
• Provide per-receiver calibration via a ref_power_offset setting to account for varying receiver sensitivity
• Provide per-device ref_power setting to allow overriding the default ref_power value.

This will allow us to account for varying beacon and receiver sensitivities caused by radio design, antenna and enclosure variations, while still allowing use of defaults for easier basic setup.

Original description:

Currently all devices use the global ref_power setting, and device names only come from local_name advertisement data, which seems unreliable but is also limiting for users, especially when doing initial setup.

Effective ref_power is a function of these variables

• device transmitter power (as well as "reported" device tx power)
• device antenna design, placement and enclosure
• receiver sensitivity
• receiver antenna design, placement and enclosure

It makes sense to be able to specify a ref power @1m value for each transmitter (device), and also an offset value for each receiver (or receiver type).

Devices:

• Allow the user to specify a name for the device (helpful if local_name isn't known, and save renaming sensors later) (no, just let them rename in the UI. Naming seems stable now).
• Global ref_power should be set to match one's most common transmitter/beacon.
• Each device can optionally have a custom ref-power defined.
• Unsure if better to define per-device setting as an offset from global ref_power or as an absolute. The latter might be less confusing.

Scanners:

• Will eventually need a setting for co-ords anyway
• A ref_power_offset makes more sense for scanners, since one is likely to have multiple identical scanners, but a wide range of devices. Also an absolute value wouldn't make sense as it's a function of the combination of tx/rx pair.
• max_radius as a per-area setting makes sense, so you don't need to enter an adjacent area in order to no longer be in "this" area.

[agittins]

Issue #164 is dependent on this being resolved.