Add default robots.txt that allows bots access to all paths.
Add mix task to generate robots.txt taht allows bots access to no paths.
Document custom emojis, MRF and static_dir
static_dir documentation includes docs for the robots.txt Mix task.
This allows to set-up an arbitrary directory which overrides most of the
static files: index.html static/ emoji/ packs/ sounds/ images/ instance/
favicon.png.
If the files are not present in the directory, the bundled ones in
priv/static will be used.
