Storing things in the session is proving buggy – we still have one user
(that we know about) where the session data isn’t getting written, so
they’re blocked from uploading a file.
Since all the info we’re storing in the session is about the file, it
makes sense to store it with the file.
This commit only does the writing of the metadata, once we’re sure this
is working we can do subsequent work to read it back, and remove
reliance on the session.
Done using isort[1], with the following command:
```
isort -rc ./app ./tests
```
Adds linting to the `run_tests.sh` script to stop badly-sorted imports
getting re-introduced.
Chosen style is ‘Vertical Hanging Indent’ with trailing commas, because
I think it gives the cleanest diffs, eg:
```
from third_party import (
lib1,
lib2,
lib3,
lib4,
)
```
1. https://pypi.python.org/pypi/isort
