Clarify how to manage the STK account

[auriamg]

We've had several questions on IRC about people who wanted to manage their account (change password, etc.) and did not know how. We should do a better job at letting people know that their account can be edited on the addons website

[Alayan-stk-2]

There is now a password change button in-game ; we need also :

• A delete account button (only available when properly logged in)
• A change username option

[apersonintheworld7]

There is now a password change button in-game ; we need also :

• A delete account button (only available when properly logged in)
• A change username option

Is the option there yet? I just play offline, so I don't need my online account and I want to delete it.

[Benau]

Sign in https://online.supertuxkart.net and delete account there in profile page