Add env variable to hide Add to MM button

[vbaranov]

Resolves #6477

Motivation

Some chains don't need "Add to MM button", e.g. default MM chains, and hiding this button shouldn't break smart-contract interaction.

Changelog

DISABLE_ADD_TO_MM_BUTTON introduced. If true, "Add to MM button" doesn't appear. Smart-contract interaction still works since chain_id and json_rpc hidden inputs moved to app.html.eex. Docs update blockscout/docs#113.

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