Reallow safe_mode=True and safe_mode='remove' markdown options

Add `safe_mode` parameter to `lib.text.render` and
`lib.text.markdown_to_plain_text` again, but use `'escape'` as default
argument.

13ab01a4488e4e2f8d396e913fee00bdb46adf88 removed these options (and
broke instance and milestone view) in order to fix
adhocracy.hhu_theme#105 after accidental removal of cgi escaping in
adf7948453bba2afcbf383bb418e160d33175907.