webapp: remove format_significant function, simplify number formatting

[Igr1x]

• Simplify format_number_short to use fixed 2 decimal places
• Remove unused format_significant function
• Canonize unit test

[AMDmi3]

Simplify format_number_short to use fixed 2 decimal places

Sorry, that's not quite what I've meant, I've meant 2 significant digits (as opposed to current 3). That would be, roughly
x.to_string() if x < 1000
format!("{:.1}k", x as f32 / 1000.0) if x >= 1000 x <= 9950
format!("{:.0}k", x as f32 / 1000.0) otherwise

9950 is due to rounding, and it would make sense to add 9949, 9950, 9951 cases to tests

[Igr1x]

Rewrote the function and added tests.
Also updated the snapshot tests repology/repology-webapp-test-snapshots#1

[AMDmi3]

Just perfect, thank you. Repository tables are now much less noisy.