MAINT: fix broken links to numfocus.org on donations page.
Merge pull request #2615 from rgommers/fix-numfocus-link

MAINT: fix broken links to numfocus.org on donations page.
fix: pass warm_start to BaseSGDRegressor
fix: make Regressor test case extend CommonTest (move two clf specifc tests to clf test case)
DOC be more explicit on mean-shift scalability

Fixes #2613.
Merge pull request #2617 from pprett/sgd-regressor-fix

[MRG] pass warm_start to BaseSGDRegressor