Premature imports in version 2.5.0

[vsemionov]

Matplotlib and Seaborn are imported unconditionally by sql explorer, even if charts are disabled in the settings. As a result, these libraries are imported every time our Django application starts up. This increases the memory consumption of our server and some long-running management commands. We only have Matplotlib installed (not Seaborn), and see about 50 MB more memory used.

Additionally, the following is now scattered in our daily maintenance logs and email reports:

[38:MainThread] [none] [matplotlib.font_manager] INFO: generated new fontManager

The above is also a result of Matplotlib initialization on a pristine filesystem.

Can we please import the chart-related libraries only if charts are enabled in settings?

[lawson89]

@marksweb @vsemionov please review this change and let me know

[marksweb]

That looks sensible to me. Thanks @lawson89

[vsemionov]

Looks good, thank you @lawson89