chore: switch to github mirrors for cloning

[Ayman161803]

Related Issue: #35

This following PR replaces gerrit with github when extensions and skins are cloned

Areas of discussion

A github mirror for SemanticDrilldown extension was not found on wikimedia's official github account. However, an alternative was found here . The same can replace current SemanticDrillDown gerrit if approved.

Changes made

Dockerfile gets built successfully after replacing gerrit with github mirrors

[Ayman161803]

cc @jeffw16 @yaronkoren

[jeffw16]

@Ayman161803 thanks for this commit. Successful compilation of the Dockerfile does not necessarily indicate the extensions are all usable. @yaronkoren do you have any good ideas on how to test these 100+ extensions still work as expected?

[yaronkoren]

Looks good. I would think just installing Canasta with this Dockerfile, and then doing an "ls -R" within the extensions/ directory would basically do the trick - plus enabling a handful of the extensions to see if they actually work. Presumably, if the same change is done for all of these extensions, then testing a few of them should lead to fairly high confidence that the rest are working as well.