ui: updated icons, converted link into button for semantic

[gorenburg]

Updated:

• Replaced PNG icons with SVG (used Material icons)
• Replaced GIF spinner with SVG spinner (used Tabler icons)
• Replaced actionable <a> elements with <button> for better semantics
• Removed outdated CSS as well as made small CSS cleanup (CSS file is 2 times smaller now)
• Introduced CSS variables for colors