model: remove BaseModel class

The only remaining purpose of this class was to provide the "sa" field,
allowing a custom SQLAlchemy session to be used for model operations.
However, this never actually worked, nor was it used anywhere.

There's always a global SQLAlchemy session associated with the current
thread; using another session for a single function call does not make
any sense (as sessions cannot be mixed), unless the code works carefully
to ensure the two sessions (and all objects loaded from them) are kept
completely separate. Suffice to say that Kallithea does no such thing,
thus there's no need to pretend to support multiple concurrent sessions.

model: simplify guess_instance invocations

The second argument to the guess_instance classmethod is in practice
fixed for Gist, Permission, RepoGroup, Repository, User, UserGroup;
so might as well move this logic into per-class specializations of
guess_instance.

model: remove docstring code example

This type of code examples are not present in other files.
It is unclear whether this example is still accurate.
And it is equally unclear whether they provide real value compared to 'real'
code.

Remove it.

model: move code from __init__.py to base.py

Having too much code, in particular too much imports, inside a package's
__init__.py is a recipe for circular imports, and considered bad practice in
Python [1]

Move out everything from kallithea/model/__init__.py to a new file
kallithea/model/base.py and adapt the existing imports.

[1] http://docs.python-guide.org/en/latest/writing/structure/#packages