Management flexibility of charts dashboard on the main page

[vbaranov]

Motivation

• Local storage keys used to display charts on the main page are not unique per chain
• There is no way to show price, market cap values under the chart, but not show show charts itself

Changelog

• Define unique local storage keys for the chart per chain by adding coin name and chain id.
• Allow to display price legend without price chart (actual for Astar instances) (manage now via SHOW_PRICE_CHART_LEGEND environment variable).

Docs update in blockscout/docs#145.

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