Make default CSS easily configurable/overridable

[lionel-rowe]

Adds classes topic_solved_title_link and topic_solved_icon and moves default CSS style to its own stylesheet instead of inline, making styles easy to customize.

Only affects phpBB 3-2+ — the phpBB 3.1 logic is unaffected.

This also adds the following minor UX improvements:

• Icons in topic list and topic title now have their own separate hover effect, making it visually clear that they lead to the accepted answer rather than the first post.
• Button icon changes to 🚫 when a selected answer is already chosen, making it clear that clicking it will cancel the existing accepted solution instead of adding a new one.