[#1429] Pass came_from to login handler after creating a user

Otherwise you lose the language setting for the UI. I'm not entirely
sure but that may involve two redirects.
[#1429] PEP8 fixes
[#1433] Flesh out the context for group/about setup_template_variables.
[#1449] Fix extras deletion

The custom-fields module was actually disabling the fields when clicking
the delete extra cross button. This prevented the key and value from
being sent, which caused a validation error.
We now change only the class to make it look disabled, which should be
good enough.
[#1449] Remove unneeded tests
Merge branch 'master' into 1449-cant-delete-extra
Merge pull request #1429 from okfn/1429-language-reset-after-register

UI language reset after account creation
Merge pull request #1449 from okfn/1449-cant-delete-extra

Can't delete extra
Merge remote-tracking branch 'origin/1433-fix-context'
Fix a Sphinx error (broken links)

@kindly