Apps menu item

[vbaranov]

Resolves #3183

Motivation

Add Apps menu item with the list of external apps relevant for the chain

Changelog

2 env vars added:
APPS_MENU - enable/disable Apps menu
EXTERNAL_APPS - the list of external apps to display in menu

Example of env vars applying:

export APPS_MENU=true
export EXTERNAL_APPS='[ { "title": "POA Mania", "url": "https://www.poamania.com/" }, { "title": "POA Bridge", "url": "https://bridge.poa.net/" } ]'

Relates PR to update the of ENV vars inn docs blockscout/docs#24

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