Allow disabling the creation of zmd files in CodiMD

[glpatcern]

A new boolean flag disablezip is defined in the config (defaults to False, i.e. previous behavior), which disables
converting a .md file to .zmd, but preserves existing .zmd files.

To be seen if we want to force converting existing .zmd files to .md.

This goes in the direction of fixing #40 though the complete fix would be to integrate the ownCloud file picker and include a public link to pictures in the markdown document.

[diocas]

Everything seems good in the PR.

But I'm not sure this is the best option conceptually... In the future, when we integrate the filepicker, do we plan to clean the DB and remove images (which are not stored anywhere else) or do we keep the images forever there? Either way is not great.

I know we cannot write to REVA to send the new file... But could we use the current prod backend and hack the opening from OCIS? Or we assume that the filepicker will arrive soon enough?

[glpatcern]

(Did not have time to discuss live, let's keep the discussion here)

My assumption is we can get the file picker quite rapidly, but let me validate this with Javier and Michal in fact ;-)
Otherwise, you have a point in that we'd need to look after the pictures stored in the storage before dismantling it, which is not great. So if the above assumption is not true, I won't merge this now.

[glpatcern]

In a second thought, I'll merge this and leave it disabled in the config. We will enable it in QA as appropriate.