Nickname and alias requirements

[tomastiminskas]

1. As a requirement for @mentions feature we will start applying some rules when setting tribe alias:

• Just LETTERS, NUMBERS and UNDERSCORE will be allowed
• Spaces and other characters won't be allowed.

2. In addition we will suggest user to match these requirements for account nickname when signing up (but it will be just a suggestion). The user will be able to set an account nickname with spaces or other characters.

3. When joining a tribe, the account nickname is used to pre fill the alias field. It will be automatically adapted changing spaces by underscores and removing special characters to match the tribe aliases requirements.