Add a search functionality to the "Add Block" popover

[ScopeyNZ]

#338 adds an "Add Block" popover. The designs show a search area that will filter down the list of blocks as you type:

Note that the designs have been updated since this ticket was created.

A/Cs

• A search box shows above the list of blocks as per the designs above
• A search box shows above the list of blocks as per the updated designs, see https://projects.invisionapp.com/share/N5MVKR392DS#/screens/318343221_Add_Block
• Typing in the search area immediately filters the list of blocks
• The "Show All Blocks" option is automatically enabled while a search query is present
• By default all available block types are shown - if no search term was entered -
• Pressing enter should not submit a parent form
• A small (x) icon appears when there is a search term - clicking it will clear the search
• A clear link appears when there is a search term - clicking it will clear the search
• After clearing the search, the previous state of "Show All Blocks" is remembered and restored.

[raissanorth]

Closed via #406