ref #770

Update .gitignore

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.

Wiki page should also be updated to recommend that admins put their custom emojis in /priv/static/emoji/custom/. This both makes for less hassle when doing a pull. And also prevents more mess in the /priv/static/emoji/ folder.