Allow separate templates for internal and external share notifications

This pr adds the ability to have separate templates for internal and external share notification emails. The templates are just duplicates of the existing mail.php and altmail.php templates at this stage so as to avoid altering the functionality of existing deployments. But they can be customised by themes.

In our test implementation we have set the text for internal notifications to direct users to their local ownCloud folder instead of a hyperlink. We found the link caused confusion resulting in the belief that shared content wasn't syncing locally. It was syncing, just so well that they didn't notice.