Verification links to other explorers

[vbaranov]

Derived from #1890

Motivation

As we should add verification in other explorers links to multiple chains, we need to keep this mockup in the master branch.

CHANGELOG

Updates relatively parent PR:

• link in the tx should lead to tx in another explorer (not to tx to address)
• links should be available for all txs/addresses (not only related to contracts)
• verification links are switched off in master by default

https://github.com/poanetwork/blockscout/blob/164d687201f0789e555c7ccc23e0116a6076bc1f/apps/block_scout_web/lib/block_scout_web/templates/transaction/overview.html.eex#L53-L54

https://github.com/poanetwork/blockscout/blob/164d687201f0789e555c7ccc23e0116a6076bc1f/apps/block_scout_web/lib/block_scout_web/templates/address/overview.html.eex#L123-L124

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

[coveralls]

Pull Request Test Coverage Report for Build 8e70d631-3d29-4b38-b0a5-889ee0a7a7de

• 0 of 5 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.07%) to 80.799%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/block_scout_web/lib/block_scout_web/views/address_view.ex	0	5	0.0%

Totals
Change from base Build 86077c4c-c7d0-48e5-83f1-f08302c14a43:	-0.07%
Covered Lines:	4755
Relevant Lines:	5885

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💛 - Coveralls