make post actions (likes, flags) translatable as whole sentences

This is a pull request following the conversation at https://github.com/discourse/discourse/pull/605. Basically, there is a lot of trouble when trying to correctly translate this:

    "You {{long_form}}"
    "You and 1 other person {{long_form}}"
    "You and {{count}} other people {{long_form}}"
    "{{count}} people {{long_form}}"
    "{{alsoName}} it too"

Although the new approach might seem worse (there is a lot of duplication), it makes correct translations possible. E.g. in Czech language, you can't have all these action in the form "{{subject}} {{did_something}}", because the spelling of subject differs for the different actions.

Note: I don't want to involve genders into this, that would be the next (and much more complicated) step.