Use hyphen separator for all SVG files

We currently have a mix of snake_case, kebab-case, and crazy_mixed-case filenames. We should make these more consistent.

Steps:

1. Add kebab-case filename requirement to README
2. Add Danger or lint rule to ensure new filenames are correct
3. Make copies of icons that have underscore names to ones with correct names (so updating doesn't break gitlab uses of camel_cased icons)
4. Remove the underscore names