Use project name in account activation e-mail

For a project named "Github", this changes the current activation e-mail from:

> Subject: JHipster activation
>
> Dear AmrMostafa,
>Your JHipster account has been created, please click on the URL below to activate it:
> ...
> Regards,
> JHipster.

To:

> Subject: Github account activation
>
> Dear AmrMostafa,
>Your Github account has been created, please click on the URL below to activate it:
> ...
> Regards,
> Github Team.

**Note**: The PR is for English only. I realize I will need to handle the rest, not sure how to collaborate on i18n though.