draw.io integration

[ssddanbrown]

Relates to #619.

Very much a WIP at time of creating pull request.

TODO

• Add updating of existing images.
• Implement within markdown editor.
• Add some basic testing to new endpoints.

[Abijeet]

This looks good to me, I'll test this on my end once you are done.

[tuaris]

This should be user configurable in case a self-hosted version is preferred.

[ssddanbrown]

At first it will not be as I don't really know how/if self-hosted instances provide the same level & ease of integration via a public url such as this. I doubt they would otherwise it could be used by anybody.

[Kdecherf]

The default self-hosted instance is a static one (check https://github.com/jgraph/drawio#running) which does not provide any integration (Google Drive, Dropbox, …). Also you must specify https=0 in query string if your own instance does not support TLS.

This lack of integration is not really important if files are saved as attachments inside Bookstack.

[ssddanbrown]

Okay, This is now complete with some basic tests to cover so will merge into master for next release.
Feel free to raise any issues for problems and bugs found, I'm sure there'll be a few I've not found.

There's a couple of limitations with the current implementation:

• For the markdown editor HTML is used to standardise the format used to recognise drawings. Think this is okay, Better than ignoring the markdown editor, but some may not like HTML in their markdown.
• Updating an drawing will update the drawing file/upload straight away, Not upon saving of the page. In the future it will probably be best to flesh this out with an image-manager-like implementation which will version drawing changes and retain old copies but what we have now is a good start.