Application alerts management: whole explorer, staking dapp

[vbaranov]

Closes #1609

Motivation

Manage Blockskout alerts

Changelog

Enables explorer alert managing through ENV variables:

SHOW_MAINTENANCE_ALERT - true/false enables announcement at the top of explorer
MAINTENANCE_ALERT_MESSAGE - Message text of the alert

Enables staking dapp alert managing through ENV variables:

SHOW_STAKING_WARNING - true/false enables announcement inside Staking Dapp
STAKING_WARNING_MESSAGE - Message text of the alert

Chore:

Add env var ENABLE_POS_STAKING_IN_MENU for management of enabling/disabling of Staking Dapp in the menu

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 53ffb62abe43bfcbc285decdda97e3f53f99e4f4-PR-3714

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.007%) to 64.424%

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Totals
Change from base Build 3684f6603d6197e54de1d0f982a9d1fb8bc9b4f4:	0.007%
Covered Lines:	2097
Relevant Lines:	3255

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💛 - Coveralls