Authentication refactoring

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes (really, really minor)
| Deprecations? | no
| Tests pass?   | ---
| Fixed tickets | #1230
| Refs tickets  | #1330, #900, #901, #1214
| License       | MIT
| Doc PR        | ---

- The **BC-Break** introduced affects auth modules auto-inserting the username in the registration form. This will not work anymore and these modules need to change one line of code. As this does not break functionality and as I cannot imagine any module using this, I introduced this BC-break.
- It is now possible for external auth modules to **hide the email adress field in the registration process** (and fill it with any email adress secretly). However the user still has to validate his email adress.
- The caps lock alert for the password fields is working again.
- Some ZikulaUsersModule javascript was simplified and refactored to jQuery.
- Some fixes in the not absolutely up-to-date (but still quite good) documentation on auth modules.
- Providing a default implementation for the authentication method selector for auth modules. No need to come with own templates anymore.
- Made it much easier to specify the login provider icon for auth modules.
- Some minor template shifiting.