CSS preload support in Firefox

[vbaranov]

Motivation

Gratefully to @goodsoft that he noticed #2514 PR disrupted modals styles for Mozilla Firefox because rel='preload' disabled in FF in the novel versions.

Changelog

Bug Fixes

Preload CSS support in FF is added with https://github.com/filamentgroup/loadCSS plugin

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. I've set master in the Version column.
• If I add new indices into DB, I checked, that they don't redundant with PGHero or other tools