Use inclusive language in signup form

[AmitMY]

trafficstars

Not all of the people in the world have a first and last name.

See https://atlassian.design/content/inclusive-writing

If you need to collect information about people’s identities, give people the option to choose not to answer, or to give a self-defined answer. When gathering information about names, use a “Full name” field instead of separating names by first name, last name, and salutation.

I recommend "Display Name" since this is the internal firebase name (displayName) but full name is a good start

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Limitations:

This PR clearly does not address languages other than English.