Correct UNI token CoinGecko ID

[vbaranov]

Motivation

Incorrect price of UNI token in xDai instance because of multiple tokens exist with UIN symbol from CoinGecko API list.

Changelog

Create a mapping to correct token id for UNI token.

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 39a975e8a8fe16f9d9f56f53f5e3244f2ebc4521-PR-3574

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-1.1%) to 66.581%

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Files with Coverage Reduction	New Missed Lines	%
test/block_scout_web/features/pages/chain_page.ex	1	90.0%

Totals
Change from base Build 74011c53dfaa6d3dc3885786f43f6c47c7c652aa:	-1.1%
Covered Lines:	2072
Relevant Lines:	3112

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💛 - Coveralls