filters should be defined as an extensionMap

[esprehn]

For consistency reasons and future proofing filters should be defined as angular.filter(name, function) instead of just assigning directly to angular.filter.

Right now it's confusing that this is the only place you directly assign while service, widget, directive, markup and the rest use an extensionMap function.

[vojtajina]

Agree on that. angular.filter() is built by extensionMap() as well, so it's only about changing the code:
angular.filter.name -->> angular.filter(name, function(){});

It's just search & replace...

[vojtajina]

Closing as duplication of #490