Image as block instead of inline

* No more inline image drag and drop in Markdown block
* Instead, provide an Image block (override Sir Trevor's) that stores
image's base64.
* Image block is considered as file, and replaced with file reference in
the Markdown file at the location of where its block stays.
* The image is rendered correctly when not viewing own entry (or not
logged in).
* Now that image is just another file, need to differentiate image and
code block for the client to render.

* Update Elixir to v0.11.0 and HTTPotion to v0.2.3