Return c_t, s_k, s_p, and s_t to hlig from dlig

[LiberalArtist]

This PR returns the ligatures c_t, s_k, s_p, and s_t to hlig from dlig, restoring the categorization from @georgd's specimen.

Related to https://github.com/octaviopardo/EBGaramond12/issues/20 and https://github.com/octaviopardo/EBGaramond12/issues/4#issuecomment-289918680.

I expect some changes will be needed before this can be merged:

• I don't have the Glyphs application. I used glyphs2ufo to generate .ufo and .designspace files, edited the features.fea files, and generated OTF and TTF formats using fontmake. If you want to keep .glyphs as the canonical format, presumably I should use ufo2glyphs to update the .glyphs files. However, I think it would make more sense to keep UFO in this repository, rather than a proprietary format.
• I have not made any change to the build script for variable fonts, which might be needed depending on the decision about UFO vs. Glyphs.
• The generated OTF and TTF files use names based on EBGaramond12 rather than EBGaramond, and the Medium, SemiBold, and ExtraBold weights are missing. Hopefully this is just a matter of passing the right options to fontmake.