Secret length

[variable]

When using django_allauth, the max_length of secret field is 100, however the generated secret string is hardcoded to 128. This commit allows configurable generated secret string lengths.

[variable]

Travis failed on python 3 interpreter not found

[palazzem]

Hi @variable,
it's ok for us to provide a configurable secret_id length for our ClientSecretGenerator.

However I can't merge this pull request cause of fc7b99a commit which is
out of pull request scope (and this is already have been merged).

So I've cherry-picked your commit. Thanks for your code!