Add HTML/ERB linting

[ArtOfCode-]

We don't currently lint our HTML/ERB code. We should add a way of doing this to bring it in line with other languages.

[Oaphi]

Note as to what to use: erb_lint looks like the standard way.

And god help us when we try) - default config, although we just have a lot of EOF errors (trailing newline):

6583 error(s) were found in ERB files

About a thousand actual issues:

1087 error(s) were found in ERB files

Down to ~600 errors without false positives:

624 error(s) were found in ERB files