Isolating of staking dapp css && extracting of non-critical css

[vbaranov]

A part of #2390

Motivation

310kB of CSS is loading synchronously before rendering of every Blockscout page

• ⬇️ 5kB of savings from Isolating of staking dapp styles. It means, 5kB less of styles will be loaded on each page of Blockscout (except staking dapp)
• ⬇️ 10kB of savings from extracting of non-critical CSS (modals, tooltips. qr-code). It means, 10kB less of styles will be loaded on each page of Blockscout. They will be loaded asynchronously.

5% of savings at all == pages should open ~50ms faster

Changelog

Enhancements

• staking dap styles is isolated in ./apps/blockscout_web/assets/css/components/stakes. Resulting ./css/stakes.css generates and is supposed to be used on staking dapp page.
• modals, tooltips, qr-code styles are extracted to critical CSS and loaded asynchronously not blocking page rendering.

Checklist for your PR

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so if necessary
• If I added/changed/removed ENV var, I should update the list of env vars in https://github.com/poanetwork/blockscout/blob/master/docs/env-variables.md to reflect changes in the table here https://poanetwork.github.io/blockscout/#/env-variables?id=blockscout-env-variables