randomize usernames for old stub participants

[chadwhitacre]

Stub participants created before 2012-09-13 have as their username the username that they have on GitHub. When we added Twitter support we changed to using randomized usernames for stub participants. We should switch to randomized usernames for the old stub participants, to release the usernames in question back into the pool and come into line with our current behavior.

But! Note that if the username starts with deactivated- then they did join but then were deactivated (manually, see #54 (comment)). If the username starts with deactivated- it shouldn't be scrambled. (Edit by @Changaco: deactivated accounts have been renamed in #2507)

Also note that we need to update username_lower along with username. Do we have a neat Python function we can use for this? We should at least use our usual random username algorithm.

Reticketed from #2362.

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[chadwhitacre]

Closing in light of our decision to shut down Gratipay.

Thank you all for a great run, and I'm sorry it didn't work out! 😞 💃