Conflict handling

[samdark]

There should be a way to deal with conflicts. A conflict is when two users are editing the same article and then saving their changes:

R = read
S = save

User1---R------------S
User2----------R-----------S

User1---R--------S
User2----------R-S

Before saving select article again. If revision isn't the same as when started editing, then we have a conflict.

We can either:

• Tell user that there's a conflict and article can't be saved (not friendly).
• Try to merge changes somehow or show diff with editor window side by side (OK?).