Write title and descriptions for app store listings

For the Google Play Store:

• App descriptions
• App title

For F-Droid:

• title.txt (app name)
• short_description.txt (short description, max 80 chars, plain text)
• full_description.txt (full app description, max 4000 chars, basic HTML allowed)

Graphic assets will be produced here: tpo/ux/design#145 (closed)