Add a config option for markdown hard wrapping

[titanous]

This patch adds a config option to gitlab.yaml that allows disabling markdown hard wrapping when rendering HTML. The relevant paragraph in the help page is wrapped in a conditional so that it will not be shown if hard wrapping is disabled.

[ecoleman]

👍 It's kind of annoying that readme's keep the breaks, as I like to keep them to the ~76 char column limit normally.

[dzaporozhets]

-1 for different behavior of gfm depends on config

[2called-chaos]

@randx I kinda agree but I have to admit that I already modified our installations to not totally mess up our moved repositories. I just removed the filtering completely but either we have to agree a more realistic configuration or we have to live with a non-portable markdown (which is one of the biggest flaws of it, every frickin page renders it different).

I don't know about the main part of gitlab users but it I wasn't able to argue why no HTML at all is allowed. This breaks nearly every markdown page with a table in it (I didn't see any markdown table for ages).

[dzaporozhets]

so the option is to disable hard_wrap or keep it as is. But -1 for configuration and diff in behaviour