suggest username when taken

[pebble-fish]

Closes #1

What this PR does

suggests an alternative username when the desired username is taken by appending values to the end of their chosen username.

Changes

• detects whether the username is taken and suggests a new username by appending a numeric suffix
• continuously suggests usernames until either 20 suggestions or a username that hasn't been taken has been found

How to test

1. register with an existing username
2. observe the suggested alternative username

[coveralls]

Pull Request Test Coverage Report for Build 21597738311

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