Add storage usage thresholds for warnings

This is just a port of
https://gitlab.com/gitlab-org/gitlab/-/blob/17a184eb15e4c4ae9e4e49119f127abe1b2b7912/ee/app/services/namespaces/check_storage_size_service.rb#L57-68

ee/app/assets/javascripts/storage_counter/constants.js

+export const NONE_THRESHOLD = 'none';
+export const INFO_THRESHOLD = 'info';
+export const WARNING_THRESHOLD = 'warning';
+export const ALERT_THRESHOLD = 'alert';
+export const ERROR_THRESHOLD = 'error';

ee/app/assets/javascripts/storage_counter/usage_thresholds.js

+import {
+  ALERT_THRESHOLD,
+  ERROR_THRESHOLD,
+  INFO_THRESHOLD,
+  NONE_THRESHOLD,
+  WARNING_THRESHOLD,
+} from './constants';
+
+const STORAGE_USAGE_THRESHOLDS = {
+  [NONE_THRESHOLD]: 0.0,
+  [INFO_THRESHOLD]: 0.5,
+  [WARNING_THRESHOLD]: 0.75,
+  [ALERT_THRESHOLD]: 0.95,
+  [ERROR_THRESHOLD]: 1.0,
+};
+
+export function usageRatioToThresholdLevel(currentUsageRatio) {
+  let currentLevel = Object.keys(STORAGE_USAGE_THRESHOLDS)[0];
+  Object.keys(STORAGE_USAGE_THRESHOLDS).forEach(thresholdLevel => {
+    if (currentUsageRatio >= STORAGE_USAGE_THRESHOLDS[thresholdLevel])
+      currentLevel = thresholdLevel;
+  });
+
+  return currentLevel;
+}

ee/spec/frontend/storage_counter/usage_thresholds_spec.js

+import { usageRatioToThresholdLevel } from 'ee/storage_counter/usage_thresholds';
+
+describe('UsageThreshold', () => {
+  it.each`
+    usageRatio | expectedLevel
+    ${0}       | ${'none'}
+    ${0.4}     | ${'none'}
+    ${0.5}     | ${'info'}
+    ${0.9}     | ${'warning'}
+    ${0.99}    | ${'alert'}
+    ${1}       | ${'error'}
+    ${1.5}     | ${'error'}
+  `('returns $expectedLevel from $usageRatio', ({ usageRatio, expectedLevel }) => {
+    expect(usageRatioToThresholdLevel(usageRatio)).toBe(expectedLevel);
+  });
+});