No strict change type validation on the client

This is related to If091211c1d1d9ab62dd04f23ec6df6cc5f3ffebb.

I really think the client should not do a strict validation of the
change type (currently 'update', 'add', 'remove' and 'restore',
defined in 4 constants in EntityChange).

We may introduce (or drop) change types at any point. Two
scenarios are relevant here:
1. A dropped change type is still present in old rows in
recentchanges.rc_params.
2. An outdated client may miss support for a new change type.

The most straightforward solution for all cases is to render
unknown change types as simple diffs. And that's exactly what
ChangeLineFormatter::format() does. It checks for a few special
cases and renders them a bit different (that's where we need to
introduce a special renderer for move/redirect actions). But no
other code outside of ChangeLineFormatter::format() should care if
a change type is valid or not, in my humble opinion.

Change-Id: I6318568ea3614eb00a38e142c307630cb4c3fa34
Merge "No strict change type validation on the client"