Add option to disable Markdown mimetype registration

[PhrozenByte]

Text has various known major compatibility issues with other Markdown software. This has been discussed extensively in the past (see #593, but also #145, #123, #439 and surely a lot more duplicates, including a whole lot more in other projects) and to cut a long story short: We can't solve this, it's how Text works and any attempt fixing this would require Text to drastically break compatibility with its previous versions - which is neither feasible nor even under consideration. It is what it is.

However, there's still an issue we can indeed solve: Text intrusively takes precedence when attempting to edit Markdown files when other editors are available. This is an issue because Text isn't just yet another Markdown editor that can be disabled if one chooses to use another Markdown editor. Text rather provides additional features, like Rich Workspace notes, and is incorporated by some other app developers to implement a (collaborative) WYSIWYG editor. Thus users might want to keep Text around, even though they don't want to use Text to edit Markdown files in their Nextcloud. Also see https://github.com/icewind1991/files_markdown#behaviors

Thus Text should add a configuration option to disable filetype registration, so that Text can be around without intervening with other Markdown editors for ordinary Markdown files. IMO we don't need a GUI option for this, something like occ config:app:set text register_markdown_mimetype --value=0 should be sufficient.