[backend] fix coding style and py3 compatibility

 - pep8
 - be python 3 compatible
 - remove old style exception raising/handling
 - use os.path.join instead of joining strings with slashes
 - use sys.stderr.write instead of print >> sys.stderr
 - use double quotes for strings and comments
 - use single quotes for single character strings

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
[cli] fix coding style and py3 compatibility

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
[frontend] fix coding style and py3 compatibility

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
[frontend] move Serializer to helpers

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
[frontend] use isdigit instead of is_int

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
[frontend] use markdown template filter

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
[frontend] use default filter instead of *_not_filled

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
[frontend] use black font color

Readability matters.

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
[frontend] put disclaimer to paragraph tags

So it is indented as the rest of the text.

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
[frontend] add margin to field label

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
[frontend] add margin to chroots-set

So it is indented as the rest of the form elements.

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>
[frontend] move 'you agree' text to dd

So it is indented as the rest of the form elements.
Remove spaces from line ends.

Signed-off-by: Richard Marko <rmarko@fedoraproject.org>