Adding the possibility to search for the blocks to insert

[youknowriad]

refs #82

This PR adds the possibility to filter the blocks menu using the search input. It also adds some scrolling to the blocks menu.

[jasmussen]

😍

Works great, thanks for doing this!

Good to merge for now, but the behavior can be finetuned further. It should be keyboard shortcuttable all the way through, as a dogma:

So, as soon as you invoke the inserter, you can use the arrow keys to pick a block and press enter to insert. So we can't set focus on the search field automatically — my bad, I added this to the prototype.

In addition to picking blocks using the arrow keys, you can start typing — even if the search field isn't focused — imaand you are filtered down to Image, with the focus rectangle now around the image. Press enter to insert the image.

To put it simply, if you're a keyboard shortcut person, you should be able to:

1. Type `/ on a new line to insert a block (see / on a newline should open the insertion dialog #24)
2. Type ima to filter down to just the Image
3. Press enter to insert the image block.

I'll open this as a separate ticket. But nice work so far!

[mtias]

Thanks! Let's ship and refine.

[anna-harrison]

This looks really good! Quick question (maybe already thought of :-) : are you thinking of indexing on fields other than the block name/descriptor and HTML code/id? As the number of blocks grows (potentially to a v large number), would it make sense to add block-type meta data to what is searchable?
Example: Show me all "media" blocks? Show me all of "my custom" blocks? Show me all "new" blocks?

[youknowriad]

Tks @annaephox for the feedback. Actually, I haven't thought about it, but it could be a good quick-win feature.

[mtias]

@annaephox yes, makes sense. I think we should add to the prototype a lot of dummy blocks just to see how it scales and feels, and whether we can quickly sort through blocks. That would cast light on where it falls short.