[feature] Add way to see version of OpenWISP modules enabled and installed

[nemesifier]

This will need a bit of thought and discussion before we can proceed.

We need a way to display the current OpenWISP version in the UI, however, right now there isn't really an OpenWISP version we can read, but every module has its own version, so maybe we could show the version of each OpenWISP module in use (but how can we dynamically determine the modules in use?).

We could have a system information page in which we show this information along other info (maybe we could show the underlying operating system).

[nemesifier]

We need a general version, the versions of the modules and some libraries that are important (we can define these).
Would be cool to have a way to show any custom module as well.
If we have a definition of what is an OW module we can do it, eg: add a special variable in the project __init__.py.

[nemesifier]

I think we can consider an openwisp module any python packages starting with openwisp-.
We can then show, additionally and if available, the versions of the following python packages:

• netjsonconfig
• netdiff

We can also show a general version number taken from the django project's __init__.py file (eg __openwisp_version__), this being optional (we will set it in ansible-openwisp2 and docker-openwisp).

The logic which fetches these version numbers must be pure python and will be called by a the UI.
The function shall return a list, starting with the general version number and the rest of the openwisp modules in alphabetical order.
The modules which are named with openwisp-, shall be listed only if they're listed in INSTALLED_APPS (watch out as the name is slightly different, eg: openwisp-controller is the name of the python package but internally in the python code is referred as openwisp_controller.config. so the code must be smart enough to figure this out.
Let's prepare another function which also returns a list with the output of the following commands:

• uname -v (OS version)
• uname -r (kernel version)
• uname -i (hardware platform)

We shall then add:

• a page which shows these version numbers, the modules first and the OS later
• a menu item with an info icon at the bottom of the menu named "System info"