Transparent coin name definition

[vbaranov]

Motivation

• Obscure definition of the coin name (it isn't managed through a single env var)
• unneeded storing of coin name in localization files in 2 different message ids.

Changelog

• Remove coin name from localization files
• define coin name it from COIN_NAME env var
• remove COIN env var from build args of docker build
• COIN env var -> EXCHANGE_RATES_COIN env vat (keep backward compatibility)

So, instead of having obscure COIN_NAME, COIN env vars and definitions of "Ether" and "ETH in .pot, .po files, the idea is to keep everything related to COIN in 2 env vars:
EXCHANGE_RATES_COIN - used by exchange rates fetching
COIN_NAME - display of coin name everywhere

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