[FEATURE] Stable links in Self hosted env's

[challapradyumna]

Is your feature request related to a problem? Please describe.
As mentioned in other issues. Stable links for diagrams.
#344
#288

Describe the solution you'd like
This solution is only for people who are willing to self-host in a safe environment. i.e people who know what they are doing.
Instead of writing to gists/IndexedDB, directly write to files in the file system on the hosted server. And for the open menu, list the files available on the filesystem of the server. Essentially stub out writing to IndexedDB with writing to the filesystem of the server. While saving, save back to the same file.

No collaboration, no user management, no fancy saving magic. If two people open the same file, whoever saves last will have all their changes.

Lightweight and can be easily switched on or off by an ENV variable when self-hosting.

Additional context