Add a flag to modified entries (with datetime value)

[alexgarel]

1. Add a flag "modified" with the last modification date to an entry (non modified entry do not have it).
2. In search enable getting all modified entries
3. Eventually also add a sort on modification date

point 2 will be handy for reviewers of a taxonomy PR. The link to this view could be added to PR description.

point 3 might be handy for anyone editing a taxonomy to retrieve last modified entries

This is halfway before tracking modifications fully.

[alexgarel]

It's done with #554, with a modification field which has a date.

Sorting in search is not yet implemented, see #291