Refactor checkExistingAddresses

[danjm]

This PR:

• moves the checkExistingAddresses from ui/app/pages/add-token/util.js to ui/app/helpers/utils/util.js, because it is used in more than just ui/app/pages/add-token/
• replaces the use of ramda in checkExistingAddresses with an ES .some call
• renames some of the variables in checkExistingAddresses to generalize its purpose
• adds unit tests for checkExistingAddresses

Note: This PR is in preparation of a forthcoming refactor of the add-token component

[metamaskbot]

Builds ready [4cfd9f4]

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

Page Load Metrics (711 ± 28 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	32	95	54	22	11
		domContentLoaded	623	863	710	58	28
		load	624	864	711	58	28
		domInteractive	623	863	709	58	28

[metamaskbot]

Builds ready [fbd5d07]

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

Page Load Metrics (675 ± 17 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	31	114	51	23	11
		domContentLoaded	623	788	674	36	17
		load	625	790	675	36	17
		domInteractive	623	788	673	36	17

[Gudahtt]

LGTM!