Append suffix to suggested username on register

[katiez667]

When a user registers with a taken username, the error message now suggests an alternative username of the form ${username}123.

Changes
Pass attempted username into the username-taken error (register.js)
Update the error message to display the suggestion (error.json)

Refs #1

[coveralls]

Pull Request Test Coverage Report for Build 21598001133

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 78.841%

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Totals
Change from base Build 21503398961:	0.0%
Covered Lines:	25358
Relevant Lines:	30328

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💛 - Coveralls