Added upload size unit selection

This is in response to [Pull Request 13026](https://github.com/owncloud/core/pull/13026).

The selection list is added and works.

It uses the existing functionality to determine which units to use when displaying the page. If the user submits 1024 MB then it will display 1 GB when loading the resulting page.

The selection list isn't perfectly aligned (it's about 1-2 px lower than the text box to the left of it). I think this is a separate issue, since it also occurs on the "settings/user" page where you choose the groups.