Allow email or username in ObjectUserProvider

[solverat]

Q	A
Bug report?	no
Feature request?	yes
BC Break report?	no
RFC?	no

Currently, only email addresses are allowed to use for login. We should allow login via username or email address. Introducing a new config flag should fix this.

Config

core_shop_customer:
    login_identifier: 'email' # this is the default value

Security

Inject parameter and switch between findCustomerByEmail and findCustomerByUsername

Improvements

• Eventlistener: Add pre-save listener to customer object: If isGuest is unchecked, check if username or email already has been taken.
• Login Form Label: Use username or email address depending on given setting (Currently we're using "username" as label, which is misleading since we actually need the email address)