styleguide: Clean up two trailing spaces
styleguide: Add a clause regarding string formatting
Fix conversion of successive Gaia plural units

When a plural unit is followed by another one, we were failing to output
the first one. Another repeated code block, but that is for the next
commit.
Refactor code repetitions
Add Gaia roundtrip script

Based on roundtrip-mozilla one.
dtd: Use source accesskey if target accesskey is missing
dtd: don't remove stray & as they probably &amp;

Since we now convert & -> &amp; we no longer need to worry about these busted
ampersands.
accesskeys: don't use space as an accesskey
PEP8: E101 indentation contains mixed spaces and tabs
typo