block contexts: MDL-19098 every block should have a context

block contexts: MDL-19098 every block should have a context
even non-course and sticky blocks.

The parent context is block_instances.parentcontextid.

The block context should be used for checking permissions directly
related to the block, like moodle/block:view or moodle/site:manageblocks.

However, if the block is displaying information about the current page,
for example the participants block showing who 'here', then it may be
better to use the context of the page where the bloack is appearing -
in other words $this->page->context - to check permissions about the
user's ability to see participants here.

Or, if the block is displaying something stronly related to courses,
for example, a course meny block, the block should probably use the
context for $this->page->course to check permissions.