FilterReview: features to be implemented

Somewhere to note down all the features that we would like:

- [ ] More speed! Faster filter calculation would allow auto-re plot.
- [x] Show filter tracking on averaged FFT plots (https://github.com/ArduPilot/WebTools/pull/11)
- [x] ~Zero pad FFT data to allow running on batch sizes that are not a power of 2. (also do some reading to check that this is mathematically valid).~ Some reading suggests that zero padding might be a bad idea ~Or make the batch length selection box move in powers of 2.~ (Done this bit) Make windows per batch option take steps such that the window size is a power of 2.
- [x] Add plot with general flight data, (roll, pitch, throttle) to allow selection of "good" portion of flight for FFT (https://github.com/ArduPilot/WebTools/pull/12)
- [x] Add param descriptions and bitmask helper (possibly auto generated from param meta data) (https://github.com/ArduPilot/WebTools/pull/30)
- [x] Add more text to page describing how it works in more detail. (https://github.com/ArduPilot/WebTools/pull/106)
- [x] Add link to filter tool for more detailed look at filter bode plots (https://github.com/ArduPilot/WebTools/pull/23)
- [ ] Support accels?
- [ ] Add loading spinner to stop dead page warnings, loading page added does not spin and will not stop dead page warnings. https://github.com/ArduPilot/WebTools/pull/55
- [ ] User configurable window overlap? Currently hard coded to 50%
- [x] Highlight the primary sensor.
- [ ] Prompt to select estimated post filter if not shown and filter param change is done.
- [x] Option to show logged notch center frequencies (https://github.com/ArduPilot/WebTools/pull/108)
- [ ] Double check ESC notch source correctly avoids dropouts. There seems to be a bug in AP's logging here, not sure we can tell dropout vs genuine 0 rpm.
- [ ] support 4th and 5th IMU
- [ ] Warn if lots of missing data

No files, tests, or entry points are named. Start by narrowing the unchecked FilterReview items into one scoped task and identifying its relevant implementation and validation points; done should mean that one selected feature is implemented and verified.

Written by the indexing model from the issue text.