NFT instance: link to the app

[vbaranov]

Motivation

Add a link to the app for NFT token if applicable.

Changelog

Find external_url property in tokenURI json and add a link to token instance page with its value

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 1a0dd2432fbde91baee7fa87bc086cff1e847848-PR-3279

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 20 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+10.6%) to 75.836%

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Files with Coverage Reduction	New Missed Lines	%
lib/block_scout_web/views/tokens/instance/overview_view.ex	20	24.39%

Totals
Change from base Build e29804af4b8a110eda68e64315fbe88e11163d06:	10.6%
Covered Lines:	1927
Relevant Lines:	2541

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💛 - Coveralls