[Admin Analytics] fix chart timezone issue

Created by: thenamankumar

• Solve charts going blank for 3 months date range due to timezone issues by moving the chart's ticks generation logic to the backend from the frontend.
• Add new events for changing calculator inputs.
• Add new events for toggling agg from totals to uniques.
• Add new events for switching date range.
• Fix font colors from black -> body.

Test plan

• Visit https://sourcegraph.test:3443/site-admin/analytics/code-intel in US timezone
• Select 3 months date range
• Verify chart showing correct values

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• Visit https://sourcegraph.test:3443/site-admin/analytics/code-intel
• Turn on dark mode
• Verfiy the legend's colour to be white