Make message rate of RC_CHANNELS_OVERRIDE configurable

#### Issue details
Allow configurable rate of message RC_CHANNELS_OVERRIDE when using a Joystick.

We are running a low latency link to an UAV running ArduPilot but the link bandwidth is limited to 22kbps (next generation Iridium). To save bandwidth we already reduced the rate of messages sent from the UAV. But when using a Joystick the GCS transmits the RC_CHANNELS_OVERRIDE message at about 11Hz from the GCS to the UAV. To save bandwidth we want to reduce that update rate.

The rate is hardcoded in MainV2.cs (Line 2299) of MissionPlanner:
float rate = 50; // 1000 / 50 = 20 hz

Its set to 20Hz but seems that is somewhat lower in a real environment due to reading the joystick inputs.

Reducing the rate in the code and rebuilding MissionPlanner worked but obviously doing that is not our preferred way since we want to stick with official MissionPlanner releases.

The ability to configure that rate from the MissionPlanner GUI - possibly within the Joystick Setup page - would be highly appreciated.

#### Version
Downloaded official release: 1.3.77 build 1.3.8110.38294
Latest from GitHub when testing the code modification: 1.3.77 build 1.3.8117.23035

#### Platform
[ ] All
[ ] AntennaTracker
[x] Copter
[ ] Plane
[ ] Rover
[ ] Sub

#### Airframe type
Quadcopter

#### Hardware type
PixHawk running ArduPilot

#### Logs
Not applicable.

Start at MainV2.cs around line 2299, where the RC_CHANNELS_OVERRIDE rate is hardcoded, and inspect the Joystick Setup page for existing configurable controls. Trace how joystick settings are stored and applied; done means the rate can be changed through the Mission Planner GUI and affects joystick message transmission.

Written by the indexing model from the issue text.