Add Etherchain links

[dternyak]

This PR introduces links to Etherchain on the following views:

1. Transaction succeeded success notification
2. Check TX View
3. Transaction History in Account View
4. Footer

I'm not completely happy with my implementation, and I spent some time refactoring the network config structure so that we could specify multiple blockchain explorers instead of hard-coding cases in the components themselves. Unfortunately, the refactor overhead was too large for these simple changes, but if this becomes a maintenance issue, a refactor to adjust the network structure to specify multiple explorer (e.g. blockExplorers?: BlockExplorerConfig[]) should be undertaken.

[coveralls]

Pull Request Test Coverage Report for Build 6283

• 6 of 8 (75.0%) changed or added relevant lines in 4 files are covered.
• 15 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.02%) to 33.492%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
common/components/ExtendedNotifications/TransactionSucceeded.tsx	1	2	50.0%
common/components/Footer/index.tsx	0	1	0.0%

Files with Coverage Reduction	New Missed Lines	%
common/components/BalanceSidebar/AccountInfo.tsx	1	1.56%
common/components/ExtendedNotifications/TransactionSucceeded.tsx	3	8.33%
common/components/Footer/index.tsx	3	31.25%
common/utils/helpers.ts	8	28.57%

Totals
Change from base Build 6276:	-0.02%
Covered Lines:	2295
Relevant Lines:	5811

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💛 - Coveralls

[skubakdj]

Code looks good IMO, and I can verify functionality.