Format strings should be unicode when value can be unicode

[michamos]

A typical cause of unicode errors is to have a non-unicode format string with a unicode value:

In [1]: def broken_format(value):
   ...:     return 'Value: {}'.format(value)
   ...: 

In [2]: broken_format('hello')
Out[2]: 'Value: hello'

In [3]: broken_format(u'καλημέρα')
---------------------------------------------------------------------------
UnicodeEncodeError                        Traceback (most recent call last)
<ipython-input-3-f592da2d7e68> in <module>()
----> 1 broken_format(u'καλημέρα')

<ipython-input-1-6aee3f447981> in broken_format(value)
      1 def broken_format(value):
----> 2     return 'Value: {}'.format(value)

UnicodeEncodeError: 'ascii' codec can't encode characters in position 0-7: ordinal not in range(128)

In [4]: def good_format(value):
   ...:     return u'Value: {}'.format(value)
   ...: 

In [5]: good_format(u'καλημέρα')
Out[5]: u'Value: \u03ba\u03b1\u03bb\u03b7\u03bc\u03ad\u03c1\u03b1'

Instead of fixing these one by one as they pop up on sentry (see #2026, #2030, #2107, #2110, #2116, #2123, #2127), it would be better to do a systematic check to solve all of them once and for all.