Drop nonexistent skins.minerva.base.reset, use skins.minerva.base.styles
The skins.minerva.base.reset module was recently merged into skins.
minerva.base.styles. skins.minerva.base.reset was providing needed styles
for heading elements in the apps, and we hadn't been including skins.
minerva.base.styles previously. This updates the base CSS endpoint to use
skins.minerva.base.styles. Dmitry tested in the Android app, and this
fixes the heading styles while introducing no side effects yet identified.