Make gr-diff-styles clearer and more specific

The 700 lines of css rules in gr-diff-styles are hard to reason about,
because they refer to all of gr-diff and its subcomponents. We cannot
move styles into subcomponents such as <gr-diff-row>, because gr-diff
components cannot have a render root. Otherwise selection across
components would break.

But we would still like to group and sort the styles better, with two
goals in mind:
1. Coming from a component you should be able to easily find the css
   rules that are relevant for this component only.
2. Coming from a css rule it should be obvious where the mentioned
   css classes are defined without searching the entire code base.

As a first step in this change we only make all the css rules more
specific, often by just prepending the relevant gr-diff component to
the rule. This was done by searching through all diff components and
looking up where each of the css class names were used. That was a lot
of manual labor. Corrected was then tested by using Dev Helper and
checking some random diffs including text selection.

We will follow up with a change that sorts these rules and potentially
breaks them up into smaller dedicated chunks.

Release-Notes: skip
Change-Id: Ie88b210dd250d86a558d225dae13bf9cc7bd49fc