Detect and block conflicting edits

Fixes the (oh so old) issue on the MantisBT site #5466, whereby
concurrent edits to a single issue can overwrite field data.

These changes allow MantisBT to spot a conflicting edit, stopping
it from overwriting the first edit with the second. It's very much
a blunt tool (flat-out refusal to save), but it works.

Signed-off-by: Damien Regad <dregad@mantisbt.org>

- Error message revised as discussed in the pull request
- Squashed commits

Fixes #5466, PR https://github.com/mantisbt/mantisbt/pull/212