add MB indicator to size heading, fix #3987

Even though it was said multiple times before, for future reference:

* We will not use multiple cardinalities, reason see https://github.com/owncloud/android/issues/189
* We will not use »MiB« because that’s »Men in Black«, not filesizes (honestly, no one knows what MiB is, but MB is well known)
* We will not add »MB« to each and every list item.

Please review @bantu @Kondou-ger @owncloud/designers