Previously Account used user id from the store, but this attribute is
optional and None by default. This caused the collection_mapping to be
unable to distinct between multiple users message collections.

This chance adds a non optional user_id attribute to Account and use it
to index the collection_mapping.

- Resolves: https://github.com/pixelated/pixelated-user-agent/issues/674
- Releases: 0.4.0

- leap_mail still uses pycryptopp and therefore still needs the
  dependency
- Keymanager calls to async HTTPClient had not been mocked, causing
  a test to fail
- fixed a pep8 warning

The key extract should check first for attached keys and if this fails
then should try the OpenPGP header.

- Resolves: #7480

Functionality that is not implemented should not fail, instead
it should be skipped until it is implemented.

Added a return to test_imap_store_fetch setup to make sure the deferred
is being handled

For #7244, this is needed for regression tests

The tests where writting their own implementation of env tear down
instead of using leap.common's one. Using it fixes many tests.

This fix stores as multi-line headers that are repeated, and that were
being discarded when storing them in a regular dict.

It had been removed during the last refactor.

I also store headers now as a case-insensitive dict, which solves other
problems with the implementation of the twisted imap.

Releases: 0.4.0