Remove iframe border

Due to the restricted user html handling in
dfa8f5e3c10f00e28af1adede2054d8d0a1cbbe9, the non-html5-compliant
`frameborder="0"` attribute, which is part of the default youtube and
other embed codes, isn't permitted by the lxml html cleaner.

Therefore iframe borders are now hidden in the default css, as nobody
wants that anyway.