Experimental activity traces

[watsonbox]

I've gone back and forward on whether to add something like this to the plugin, and if so on which approach to take.

My intention was to create markdown notes from Strava activities, so they could be linked to and made part of some basic Obsidian reporting workflows. I didn't want to end up using Obsidian as a backup for Strava data: there are other tools for that job. Nor did I want to replace any of the first-class tools and data analysis functionality of the Strava app itself. That's a link away if required. From that point of view I never really planned to add activity trace maps, and I'm still not sure it's the way to go.

That said, I can see how being able to visualize the activity trace itself might be useful, and considered two approaches:

1. Using the obsidian-leaflet plugin
2. Including the Leaflet library directly

After trying both, this experimental PR takes the first approach: primarily to avoid having this plugin itself become a map rendering tool. It does come with drawbacks, though:

😞 The obsidian-leaflet plugin has a bug preventing it from zooming correctly to the loaded geometry by default.
😞 The plugin isn't able to display geometry from front matter (e.g. polylines), meaning that a separate file is required.
😞 Separate geometry files aren't handled by Obsidian so for example links are broken if they're renamed.
😞 obsidian-leaflet is in in maintenance mode, and there aren't any other Obsidian plugins capable of displaying traces.

All this supports my original intuition: Obsidian probably isn't the right tool for visualizing geometry.