(#3063) part 2: Hide blocktypes that don't make sense when making a
View. This means that blog/profile/resume blocktypes don't show up in group/institution/site Views. This is controlled through a new Blocktype method - allowed_in_view. Various code had to be changed to listen to this method, mostly around the display of the category browser. When making a template, all blocktypes are available. Part 3 is to change the configuration forms to be more sensible when editing a template, and then testing what happens when blocktypes are rendered somewhere that they "shouldn't" be - e.g. a My Friends block in a group View.