Gas usage at main page

[vbaranov]

Motivation

Display total gas usage at the main page

Changelog

Checklist for your Pull Request (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 by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build 011ee7e46eda6f8fef072893c24ae114940f80e3-PR-3385

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 7 unchanged lines in 2 files lost coverage.
• Overall coverage decreased (-1.3%) to 67.479%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	2	71.43%
lib/block_scout_web/controllers/chain_controller.ex	5	86.84%

Totals
Change from base Build cb6076907d29c97608e53e51b55beb3b7c8a0a89:	-1.3%
Covered Lines:	2048
Relevant Lines:	3035

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💛 - Coveralls