Actually honour the concept of `custom_back_text`

When I set `custom_back_text`, I expect the back button to only contain that text, not something prefixed with what someone else believes is a good piece of text.

These non alphanumeric symbols that are there for affordance, can be replaced with css3 and font-icons.
Merge pull request #3271 from airtonix/patch-4

Actually honour the concept of `custom_back_text`