Markdown block to render and save image's base64.

When loading data, the blocks replace any reference to a base64
image with the file's name specified in the `alt` attribute. It stores
the file's base64 into the map.

Upon receiving a file on drop, the block calculates and store the file's
base64 in a map by its filename.

On submit, it replaces any image with `src` of its name with the right
base64 content and set the `alt` attribute with the filename.

For now, the base64 content is save directly in the content of the file.

Known bug: Upon submitting, the POST redirects to the image file.
Redirects to the gist after editing instead of `prev_path`
[WIP] Render images after entry is loaded, logged in or not

* If not logged in, render the image directly from base64.
* If logged in, convert file references that are images to `<img>` tags
with base64 content. The markdown block will convert it to filename,
as well as convert it back to base64 on preview.
* Upon submitting, the base64 content converted by markdown block
is stored into a file and replaced with a file reference, just like any other
files.

Right now, it fails to submit, but too tired to debug it.