Added exponentially smoothed trendline for weight

[seth-leavitt]

Proposed Changes

• To improve the impact of each weight measurement, I've added an exponentially smoothed trendline.
• Individual measurements are shown as plotted points.
• The points have their variance from the trend plotted from the point to the trendline for easy interpretation.

Related Issue(s)

If applicable, please link to any related issues (Closes #123,
Closes wger-project/other-repo#123, See also #123, etc.)

• No Issues of note

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)

[seth-leavitt]

Using a longer term smoothed charting allows for easier and faster interpretation of how your actual body weight is changing rather than just simply seeing variation in the weight plot.