Tidy up block CSS in the clean theme

As clean nears becoming the default theme we need to consider tidying up the Block CSS as there are a couple of issues within it.
The general layout could also be improved, making the most of less to clarify block structure for those who are looking to re-theme or modify.

Issues noted:

• `.block .header h2` and `.block .header .title h2` style the same element, conflict and one is certainly redundant.
• `.block .blockannotation` is incorrect, the blockannotation class is not used within .block. This can be removed as its clearly not used.
• `.block_calendar_upcoming .footer` and `.block_calendar_upcoming .content .footer` style the same element as the footer is always rendered within the content div.
• `#searchform_search` is not specific enough, it conflicts with the styling of an element with the same ID within the tag content. It should be related to the search_forum block for which it is designed to style.
• Padding on `.block-hider-hide` and `.block-hider-show` is unnecessary as `.block_action` padding covers these icons.

This issue has been marked as a bug because of confusing redundancy that should be fixed in stable themes.