Post-release UI cleanups

[gcampax]

After #977, there are some left-over UI issues that are worth polishing:

• font sizes are inconsistent and often wrong (see for example in Commandpedia, secondary text is bigger than primary text)
• colored and plain buttons are inconsistent, for example in Thingpedia device pages
• the X button in the conversation widget is broken when the mic button is used
• spacing in the "My Skill" panel is inconsistent
• there is a lot of duplication between styles for the anonymous mode (in the index page) and styles for the logged in conversation, with some styles in style.css and some in conversation.css - this is messy and can lead to bugs

We should clean up all of these, apply them to the stable branch, and release as 2.0.1 soon.
A separate issue will be filed to track major UI changes for the next development cycle.