Add search collectibles modal

[unjapones]

Closes #325.

Includes

• Open a modal to search through the list of all assets in the redux state (search performed on name and ignored-case substring match). To open the modal, the user should click on the search input in the erc721 toolbar subapp.
• Make collectible_card a non redux-connected component and turn it a link/anchor (uses Link from react-router-dom
• Behavior was deduced from Figma's screenshots.
  • when is typed on the search bar, nothing is shown in the results.
  • when something is typed the results will appear accordingly and the text "Showing NN results" will render accordingly.
  • selecting a filter on the top-right corner affects the visibility of the results, but not the search result set (then, the text "Showing NN results" will not change).

[unjapones]

The history of this PR/branch is a little bit weird, we might squash this when we decide it is finally ok.