Makes the global search system work with blocks on each user's dashboard.

(I'm adding this mainly just because I've written the code - I'm not sure it should actually be implemented at this stage. Unless critical, I don't plan to work on this but am happy for somebody else to take it on.)

The main disadvantage with this change is that it does not work properly for the admin user. Specifically, if the admin user searches for something that happens to exist on some user dashboards, it will give results in /my/index.php but these links do not work because they are specific to the user involved. (The search system currently doesn't have a mechanism to restrict admin contexts, and there is no clearly-a-good-idea way to remove these results after the fact either.)

The effect is particularly bad if you add an HTML block to the default dashboard, then if that has the word 'frog' and the admin searches for 'frog', and there are 100,000 users on the site, they will get 100,000 results from user dashboard blocks they can't access.

Other than for admins, it works OK (probably not fully tested mind)..

Note: The diff URL currently is showing the full diff including MDL-59857 changes; obviously it is only the last commit that is for this issue.