Remove ./assets/dir-index-html submodule

[lidel]

I believe we should merge dir-index-html back to go-ipfs.

No reason for it to be outside. It is a key part of gateway functionality, and having a separate repo + git submodule makes it ridiculously hard to contribute changes.

[longfeiWan9]

@lidel li maybe I can try to work on this as part of BootCamp?

[lidel]

@longfeiWan9 sounds good!

Some pointers:

• would be really nice if we could preserve git history while replacing ./assets/dir-index-html submodule with a plain subdirectory, to keep contributor info intact
• (optional research) after move is done, you could see if we still need the bindata build step in ./assets:
  • afaik BindataVersionHash is used only here in the Etag HTTP header to indicate to the browser when HTML with directory listing changed
  • we also use bindata to generate something for ./assets/ init-doc but i'm lacking context why (no need to change anything, just curious myself :))

ps. i'm around if you want to chat about it (calendly link is in my slack profile)