🛠️ Refactor: use vue-svg-loader for all SVG icons

[dlqqq]

Motivation 🏁

vue-svg-loader is currently being introduced in PR #83. It treats all SVG files as Vue components, and calls svgo in the build step to optimize all SVGs, greatly improving load times. This means cleaner code (no more disgusting 10-line long SVGs in the middle of Vue components) and faster load times for the end-user.

Describe your refactoring solution 🛠️

• identify all SVG assets used (e.g. difficulty icons in SVG Refactor + Icon Hotfix #83, donation icons in Add donate dialog #79)
• replace them all with separate SVG files and use vue-svg-loader in all component files