Reduce horizontal spacing in the New Source Browser view

###Problem Definition###
There is an unnecessarily long horizontal spacing in the new Source view for a repository, resulting in being required to use a horizontal scroll bar to scroll all the way to the right.

This can be observed with different browsers as well as with incognito mode.

If the New source browser experience is turned off in Bitbucket Labs. the old Source view displays the page with the proper spacing, resulting in there being no need to scroll horizontally.

###Suggested Solution ###
Reduce the horizontal spacing.