Improves error handling, add missing header to __init__.py and remove
mkdirs from flags methods. This is a commit from code review on !117.

IncomingBox spec has a flags feature for the processing flow of
messages. This commit adds it using a .flags file.

-- Resolves: #8869

Refactor suggested from !105 review.

Incoming API will receive externally encrypted PGP documents. This
commit adds this kind of encryption scheme and method to preamble module
on soledad.common

-- Resolves: #8890

This move allows server to use it on #8868 as described in #8890

-- Relates: #8890

We have been discussing about this merge for a while.
Its main goal is to simplify things: code navigation, but also
packaging.

The rationale is that the code is more cohesive in this way, and there's
only one source package to install.

Dependencies that are only for the server or the client will not be
installed by default, and they are expected to be provided by the
environment. There are setuptools extras defined for the client and the
server.

Debianization is still expected to split the single source package into
3 binaries.

Another avantage is that the documentation can now install a single
package with a single step, and therefore include the docstrings into
the generated docs.

- Resolves: #8896