Write contracts feature

[vbaranov]

Resolves #71
Resolves #72
Resolves #73
Resolves #77
Resolves #80
Resolves #81
Resolves #3170

Motivation

Blockscout doesn't provide the opportunity to send transactions to smart-contracts

Changelog

• a new tab with write-only functions
• interaction with MetaMask/Nifty Wallet
• success/warning/error modal windows (styles are from staking dApp PR Add Web UI for POSDAO Staking DApp #2292)
• support payable methods
• enables "Write proxy" feature
• excludes constructors from write functions list

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 1e34a1b2-3b6e-42eb-b479-c001357141b0

• 56 of 67 (83.58%) changed or added relevant lines in 10 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 74.38%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/block_scout_web/lib/block_scout_web/views/address_read_contract_view.ex	1	2	50.0%
apps/block_scout_web/lib/block_scout_web/views/address_read_proxy_view.ex	1	2	50.0%
apps/block_scout_web/lib/block_scout_web/views/address_write_contract_view.ex	2	3	66.67%
apps/block_scout_web/lib/block_scout_web/views/address_view.ex	2	4	50.0%
apps/block_scout_web/lib/block_scout_web/controllers/smart_contract_controller.ex	11	14	78.57%
apps/block_scout_web/lib/block_scout_web/views/address_write_proxy_view.ex	0	3	0.0%

Totals
Change from base Build 3caf8ffc-1bef-4193-8b28-e784217923bb:	0.05%
Covered Lines:	5966
Relevant Lines:	8021

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💛 - Coveralls