Network icons preload

[vbaranov]

Motivation

Network icons load every time when a user opens the network menu causing for a while:

Changelog

Enhancements

Preload of network icons added with this PR

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

[coveralls]

Pull Request Test Coverage Report for Build e6051c43-adb2-49e3-a050-8a4cfe3e33da

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 79.171%

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Files with Coverage Reduction	New Missed Lines	%
apps/block_scout_web/lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	1	78.57%

Totals
Change from base Build 40ef733c-0c96-4391-964b-9a12e1e93cb8:	-0.02%
Covered Lines:	5158
Relevant Lines:	6515

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💛 - Coveralls