Fix for "The alias option of the form.type_extension tag is deprecated"

[Alex-Sh]

Please fix following deprecation:

Message:
The alias option of the form.type_extension tag is deprecated since version 2.8 and will be removed in 3.0. Use the extended_type option instead.

File:
https://github.com/braincrafted/bootstrap-bundle/blob/develop/Resources/config/services/form.xml

[dkisselev]

#409 is where we're currently discussing the 3.0 deprecations, looks like this one's already been mentioned in there and someone said they're working on a PR to clean up a lot of the known remaining ones.