Various CSS fixes

[gravitystorm]

This PR fixes a few CSS issues that were highlighted in #2492 and others that I noticed while working on that. In summary:

• Uses bootstrap's list-group-flush to replace custom css, for sidebars with lists-with-lines-between
• Uses h6 for the changeset comment tag text
• Removes some unnecessary custom css rules
• Tweaks the rss icon to line it up with other icons

[tomhughes]

I've added one extra tweak in ffb671b to pull the query tool results back to the left as this change had, probably unintentionally, led to them being indented like this:

My change moves them back to the left in line with where they were before:

The geocoder search results could probably do with being converted to this new format as well...

[gravitystorm]

probably unintentionally

It was actually deliberate, but I'm not too fussed either way :-) I think the horizontal lines that stretch beyond the left-most character of the heading makes it look like the heading is part of one of the items.

  heading

  stuff
---- divider ---
  stuff

  heading

  stuff
---- divider ---
  stuff
---- divider ---
  stuff

The above looks like a single list of four items with three dividers to me.

  heading

    stuff
  ---- divider
    stuff

  heading

    stuff
  ---- divider
    stuff
  ---- divider
    stuff

The above looks more like two lists, with a header before each list and dividers between the list items.

But like I say, there's perhaps bigger things to worry about! Just thought I should explain my reasoning.

[tomhughes]

I think it was the lines sticking out beyond the text that I found odd - the actual indentation wasn't particularly unreasonable.

Actually I think it's the combination - the lines sticking out would work if all the text was indented but having some text align with the lines and some not winds up looking a bit weird to me somehow?