SVG network icons

[vbaranov]

Motivation

Icons are vague on Retina displays

Changelog

• SVG images are added for networks selector
• PNG images are removed
• CSS is changed to use SVG images

BEFORE:

AFTER:

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 caa53a49-157b-4aa6-8d60-3d3b0ac23870

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.996%

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Totals
Change from base Build e37df190-7246-494d-9d68-7b147df7add3:	0.0%
Covered Lines:	5194
Relevant Lines:	6575

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💛 - Coveralls