Before the change campaign bonus selection screen had inconsistent
overflow behaviour for difficulty selection:
```
0 1 2 3 4 | available buttons: '-' to decrease
^ | '+' to increase
```
Before the change:
1. If we click '+' 5 times we will end up on difficulty=4 (ok, saturated).
2. If we click '-' 5 times we will end up on difficulty=1 (unexpected, wrap around).
After the change:
1. If we click '+' 5 times we will end up on difficulty=4 (saturated).
2. If we click '-' 5 times we will end up on difficulty=0 (saturated).
The inconsistency happens because `difficulty` variable has `ui8` type
and server uses `difficulty = vstd::abetween(difficulty, 0, 4)` to
implement the saturation.
For large positive values saturation works as expected:
vstd::abetween(difficulty=5, 0, 4) -> 4
For small values it does not:
vstd::abetween(difficulty=-1, 0, 4) -> 4
The change makes client to avoid using negative values.
e407d4e547