Add profanity filter to autogenerated shortlinks (for #1174)

[jace]

This PR adds a profanity filter to auto-generated shortlinks using better_profanity. This package has a performance problem in the current release 0.7.0 as described at snguyenthanh/better_profanity#19.

Performance test:

$ python -m timeit -s "from funnel.models.shortlink import random_bigint, bigint_to_name; \
from better_profanity import profanity; \
cf = profanity.contains_profanity" -- \
"cf(bigint_to_name(random_bigint()))"

With the current 0.7.0:

500 loops, best of 5: 679 usec per loop
500 loops, best of 5: 672 usec per loop
500 loops, best of 5: 700 usec per loop

With the previous 0.6.1:

50000 loops, best of 5: 10.8 usec per loop
20000 loops, best of 5: 9.9 usec per loop
20000 loops, best of 5: 11.4 usec per loop

Therefore we have the dependency set to better_profanity<0.7 until this issue is fixed.