Add Sentry performance monitoring and environments

[taneliang]

1. Performance tracing: adds the basic Sentry performance setup. The most valuable data here is the web vitals stuff, but it'll also be the foundation of future tracing for the new GraphQL server as well.

   I've added a Timing-Allow-Origin header to api.nusmods.com so that we can see timing info for it in our traces.

   Sentry do provide a React integration that can log basic React lifecycle info for components wrapped in a withProfiler HoC. However, lifecycle info (i.e. mount, render, update, unmount) for single components isn't very useful, and you'll need to wrap each component of interest individually. I think it may be more useful to build our own React tracer using the user timing marks I helped add to React, which are enabled in experimental profiling builds of React, which will be able to provide timing information on React state updates, suspends, etc, as well as info on effect execution and render and commit phases.

2. Environments: adds some heuristics to determine an environment.

Screenshots

All links are internal-only.

Environments

Performance tab

Transaction details

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[codecov]

Codecov Report

Merging #3106 (4278a1b) into master (5268d13) will decrease coverage by 0.07%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3106      +/-   ##
==========================================
- Coverage   55.58%   55.51%   -0.08%     
==========================================
  Files         255      255              
  Lines        5309     5316       +7     
  Branches     1218     1223       +5     
==========================================
  Hits         2951     2951              
- Misses       2358     2365       +7     

Impacted Files	Coverage Δ
website/src/.eslintrc.js	0.00% <ø> (ø)
website/src/bootstrapping/sentry.ts	0.00% <0.00%> (ø)

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[nusmods-deploy-bot]

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