Add exponential moving average to measure habit strength

[lashoun]

Description

This solves https://github.com/glutanimate/review-heatmap/issues/84 and also changes the formula of the daily average number of cards so that it also uses an EMA, which should be more comparable to current reviews.

Checklist:

• [x] I've read and understood the contribution guidelines
• [x] I've tested my changes against at least one of the following Anki builds:
  • [x] Latest standard Anki 2.1 binary build
  • [ ] Latest alternative Anki 2.1 binary build
• [x] I've tested my changes on at least one of the following platforms:
  • [ ] Linux, version:
  • [x] Windows, version 2.1.54:
  • [ ] macOS, version:
• [ ] My changes potentially affect non-desktop platforms, of which I've tested:
  • [ ] AnkiMobile, version:
  • [ ] AnkiDroid, version:
  • [ ] AnkiWeb