Allow specifying optional tags for blocks

In #2324 we have been trying to address the problem where the search in the "Add block" dialog would not include all menus when a use types menu.

The search in that block checks both the block title and the block description; so the obvious solution for menus that do not include the word "menu" in their titles, was to add this word to the menu description. We initially went with A list of links for the %menu_name% menu, but if %menu_name% already included the word "menu" in it, you'd end up with duplication of the word "menu" for those blocks (for example "A list of links for the Administration menu menu").

After a few back and forths, we settled with A list of links for the menu: %menu_name%, which works better (although you may still end up with duplication of the word "menu").

This seems to be a generic issues where you could end up with descriptions like "A custom block block" or "My view view" etc., so I would like to explore the idea of adding tags to blocks. These tags would be taken into account when searching in the "Add block" dialog, but do not need to be part of the block description.