Allow interaction with navbar menu at block-not-found page

[vbaranov]

Motivation

The same issue as here #3192 but for "Block not found" page.

Changelog

Add app.js to "Block not found" page.

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

[github-actions]

Pull Request Test Coverage Report for Build 52c3ce52324a52ce9d4251d793be7c7b0097b2aa-PR-3220

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-16.4%) to 55.682%

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Totals
Change from base Build e3810852aff2ead7c88dc0ef874540c70fd23c27:	-16.4%
Covered Lines:	441
Relevant Lines:	792

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💛 - Coveralls