Add support for uploading Avatars [#IDN-1774] mk2

# Avatar upload

![avatar](https://cloud.githubusercontent.com/assets/1170410/2720958/1694cbc2-c576-11e3-9dd3-8cf71713ef61.png)

Part of our r2 Feature Parity series: This pull request carries on where @rtyley left off with the avatar upload work - we now have a full working flow.

See original pull request here and its description below: https://github.com/guardian/frontend/pull/3853

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Allows users to upload new Avatars for their profile:

https://profile.thegulocal.com/public/edit

The avatars are sent for moderation in the Discussion tool, so they're not available immediately.

The Avatar upload system we're hooking into /could/ be described as legacy. The fact that it uses GAE is not the problem, the dependency on the Guardian DocRoot (rather than S3) is closer to the mark.

https://jira.gutools.co.uk/browse/IDN-1774
https://sites.google.com/a/guardian.co.uk/discussion-project/
https://sites.google.com/a/guardian.co.uk/discussion-project/avatars/avatar-upload

The Avatar upload is behind a switch, which is off for now.

![be8cc1ea-c08f-11e3-95a2-f17ac0f77c3b](https://cloud.githubusercontent.com/assets/1170410/2720967/2c2258ec-c576-11e3-9369-bfc495e26492.gif)