rename Ethereum Main Network -> Etherum Mainnet

[brad-decker]

Replaces the phrase 'Etherum Main Network' with 'Ethereum Mainnet'. I treated this as a proper name, and replaced it in all places it existed in all translations as well. This does result in NETWORK_NAME_MAP from enums.js but it doesn't seem widely used.

On Develop

This PR

[metamaskbot]

Builds ready [ceef2ab]

• builds: chrome, firefox, opera
• bundle viz: background, ui, inpage, contentscript, ui-libs, bg-libs, phishing-detect
• dep viz: background
• all artifacts

Page Load Metrics (502 ± 78 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	30	113	49	26	13
		domContentLoaded	260	807	501	162	78
		load	261	810	502	162	78
		domInteractive	260	807	500	162	78

[rekmarks]

LGTM!