Consolidate Create and Place Content tabs into one tab

This patch consolidates the Place and Create Content tabs into one tab, "Manage Content".

This was done for a number of reasons:

1) There is lots of overlap between the tabs - both display forms, both interact with Blocks, and both display the same sort of information.

2) When the CTools Block UI is implemented, there will only be one place to interact with Blocks - and it will look similar to this (albeit not in the tabs view and using vertical tabs instead of horizontal ones).

3) This change reduces code overall which should help in tasks involving porting this code to a backend UI.

4) I can start working on adding Blocks from regions now, which is line with our UX goals (and is a functionality gap at the moment). If we kept two tabs every "Add to region" area would need to be split into "Create here" and "Place here", which would have been awkward.