Log of edits to competitions.

[AlbertoPdRF]

As suggested by @Laura-O, we should have a log of edits to competitions, with the following features:

1. Show a "Last edited by XXX, on Month Day, Year, Time" message at the bottom of the edit competition page.
2. Store an historical copy every time something is changed.

[LinusFresz]

I think that's at least partially related to a 'competition action log' requested in https://github.com/thewca/worldcubeassociation.org/issues/2335.

Edit: sorry for closing.

[cubewhiz]

Where does this issue fall in the priority queue?

[AlbertoPdRF]

This is tangentially related to #4219.