Enhancement: parallel use of markdown file: warning if a newer version exists

[hofmannc]

Testserver: qa keeper
Browser: firefox 77.0.1 (64-Bit) Chrome 83.0.4103.61
Version: Seafile 7.1.3

Action:
User 1 makes changes in markdown file
User 2 enters same markdown file

User 1 > "Save"

User 2 -> makes changes -> "save"

Observation: Changes of user 1 are overwritten

Proposed situation: User 2 gets a warning that he is about to overwrite other user's changes.