Allow selecting language optgroup in filters

[Xyene]

If we make the optgroup the language common name, we can have a select2 filter that looks like:

C++
    C++11
    C++17
Python
    Python 2
    Python 3
    PyPy 2
    PyPy 3
...

...and selecting the "Python" header would select all the children. Seems easy enough with select2. Requested by a user via email.

[tanweeralii]

Can u assign this to me?